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Endangered and Threatened Species of the Platte River (2005)

Chapter: 4 Scientific Data for the Platte River Ecosystem

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Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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4
SCIENTIFIC DATA FOR THE PLATTE RIVER ECOSYSTEM

Background discussions in Chapter 2 and the analyses of environmental law and science in Chapter 3 lead to an assessment of our understanding of the ecological foundations for threatened and endangered species in the Platte River Basin. The agencies of the U.S. Department of the Interior (DOI) have evaluated research into the habitat requirements of the whooping crane, piping plover, interior least tern, and pallid sturgeon for survival and recovery and have based their recommendations for instream flows and river management on the relevant studies. Critics question the studies.

The charge to the committee regarding the Platte River ecosystem and its management generally takes the form of assessing the “scientific validity” of DOI decisions. For the purposes of this report, a management decision and the conclusions that led to that decision have scientific validity if the scientific knowledge that existed when the decision was made is identifiable and verifiable and was the product of research methods and techniques that were generally accepted by the scientific community at the time of the decision. The data and the information that resulted from processing them must be identifiable and archived so that they are recoverable by subsequent investigators if needed. The conclusions drawn and the theories and models used to understand and explain the conclusions must be verifiable, that is, subsequent investigators must be able to replicate the research and arrive at the same conclusions. The methods and techniques must be similar to those used by other workers in similar applications and be commonly found in the scientific literature or discourse of professional

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

meetings. The committee assessed the science used by DOI agencies as it existed when the agencies made their decisions. Since those decisions were made, there have been advances in science and engineering that may improve management.

This chapter evaluates the types, relevance, and quality of science used by DOI to understand and manage the individual endangered species and the ecosystem associated with the river, and it assesses the validity of the science for policy decisions. The scientific basis of listing a species and designating critical habitat is discussed in Chapters 5, 6, and 7. This chapter begins with a consideration of how science is connected with the goals of restoration of the Platte River for the benefit of threatened and endangered species. It then describes the basic connections that sustain the Platte River ecosystem (including its hydrology and geomorphology) and the habitats important for its threatened and endangered species. Next, it addresses specifically the validity of the science underlying DOI decisions related to instream flows and ecosystem connections that managers use to preserve and enhance habitat for threatened and endangered species. The chapter concludes with some special scientific considerations for decision makers that have not yet been fully explored.

SCIENCE AND MANAGEMENT TARGETS FOR SPECIES

Management of the Platte River for the benefit of threatened and endangered species entails a preliminary decision that deeply involves science and the state of our knowledge. It is likely to require restoration of the physical system of the river, its hydrology and geomorphology, to create habitats useful for sustaining the species. Restoration in this sense implies managed and designed changes to alter the existing river to some other target condition. The target of restoration in most applications is the presettlement condition because those arrangements supported in relative abundance the species that are now endangered or threatened. It is rarely possible to completely attain such a restoration because of human effects such as land-use changes in the watershed and water-control infrastructure, but as a general objective the presettlement conditions represent the end-point of a spectrum of possibilities. There are two fundamental approaches to restoration: first, through knowledge of the presettlement conditions, and second, through knowledge of the present connections among physical systems, habitats, and species. In the case of the Platte River, restoration of the river to its prehuman or pre-European-settlement condition is faced with three issues: first, knowledge about prehistoric systems is sparse; second, they were always changing; and third, it is not possible to reconstitute them. First, knowledge about the prehuman ecosystem of the Platte River is highly limited by the lack of direct observations. Proxy measures of the

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

environmental conditions are sparse because of the general dryland nature of the northern Great Plains. Although geologic evidence can inform us about the long-term general environments and their adjustments on a scale of tens of thousands of years, the evidence is not detailed enough to reconstruct specific environmental conditions along reaches of the river a few kilometers long. These issues commonly face restoration efforts, and they are not unique to the Platte River case.

We have considerable knowledge about the nature of the Platte River system’s general characteristics for the period between the arrival of early settlers and the twentieth century. By the time of official Government Land Office (GLO) surveys, the most exacting early assessments of landforms and vegetation that were conducted in the 1860s, substantial alteration of the vegetation by Europeans had already occurred. By the 1890s, the water system was also subject to human-induced adjustments. As a result, there are three distinct time periods of our knowledge base: the presettlement period for which we have little direct information during which the now listed species were abundant; the post-settlement period of the nineteenth century for which we have some knowledge during which the species were under considerable pressure, and the twentieth century period for which we have the most information when the species populations declined to very low levels.

Restoration of the central and lower Platte River ecosystems to their presettlement conditions is not possible, even if the prehistoric target conditions could be specified. The central river and lower river are at the downstream end of a far-flung drainage basin and river system, and they exhibit characteristics that are determined by processes throughout the watershed. Land use and land cover are now substantially different from the prehistoric conditions, and the watershed hosts several large dams and many smaller control structures. Those features change runoff and stream flows and could not sustain a prehistoric reconstruction in the central and lower Platte River.

A second approach to defining restoration goals (in addition to knowledge about undisturbed ancient conditions) is to establish the sorts of conditions that we know from research in present environments favor the threatened and endangered birds and fish but are also consistent with our knowledge of presettlement conditions. We can then create an environment that contains those conditions (Box 4-1). This approach has the advantage of working from an observable premise: the connections among river flows, geomorphology, vegetation, and wildlife. Those connections are complex and are not completely understood; but given our partial knowledge about them, restoration for species is possible. As this normative approach to restoration proceeds, corrections and adjustments, particularly in the flow regimes of the river, can provide for experimentation through adaptive management.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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BOX 4-1
Restoration for the Future Platte River

The primary route for managing the Platte River to benefit threatened and endangered species is to “restore” the river flows. The basic hydrological question, however, is to restore the flows to what target condition? One choice might be entirely “natural” flow regimes, such as ones that might have existed before the arrival of humans or at least before the imposition of numerous water-control structures that occurred after European settlement.

It is not possible to return the flow regime of the Platte River to either of those “natural” models, for three reasons: knowledge, limitations of the present system, and social considerations. First, we lack detailed knowledge about the true nature of prehuman flow conditions, and we have only minimal understanding of flows before the construction of dams and diversion works. We can compensate somewhat for this lack of direct knowledge by application of theory. For example, we know how modern braided rivers work, and we know the prehistoric Platte River was a braided stream, so we can make some useful generalizations about the presettlement river. Second, the present hydrological system includes widespread changes in hydrology throughout the watershed. For example, widespread intensive grazing of many areas may or may not mimic grazing intensity and patterns of the original native animals that once roamed the region. Mountain watersheds now include storage reservoirs that did not exist in presettlement periods. Therefore, even if adjustments could be made for some “natural” target in the central and lower Platte River, those adjustments are not likely to be easily coupled with changed runoff and storage conditions upstream of those reaches. Finally, the water-control infrastructure of the river is in place as a result of public decision processes that sought to create the present hydrology, characterized by suppressed flood peaks and a dependable water supply even in dry months. A complete return to the presettlement flows would mean the abandonment of social and economic goals that have driven substantial investment in the system—an unlikely scenario.

A more likely alternative for the target of flow restoration is the blending of objectives, whereby some flow characteristics benefit key wildlife species and attempt to mimic presettlement conditions to the extent possible. Other hydrological characteristics also remain in place to serve additional (such as social) needs. Such a compromise arrangement represents a more “normalized” flow regime that mimics natural rhythms, magnitudes, and durations but within constraints that recognize the changed nature of the basin and other competing economic demands on the water resource. In that way, restoration of the river flows is restoration part of the way toward the “natural” objective.

Any restoration and management program will have to go forward with incomplete knowledge, but decision makers can use the best available scientific knowledge in guiding their choices. Researchers and decision-makers can provide education for a public that may expect higher levels of scientific certainty than are possible. Ecosystem research is not as controlled or exacting as a bench science, so its input to public decisions is accompanied by more uncertainty than is the case for laboratory sciences.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

BASIC CONNECTIONS IN THE PLATTE RIVER ECOSYSTEM

Issues defining the current conflicts in the central Platte River are related to seven interconnected elements: threatened and endangered species, other species (not regulated by the Endangered Species Act), flow of water through the river, sediment transported by that flowing water, groundwater, agriculture (and other human systems and uses), and riparian vegetation. Habitat is controlled by those elements, and they are intricately linked by connections that ultimately respond to stream flow and other ecological processes (such as fire, grazing, and human use). Water flowing through the river constitutes mass and energy, and changes in stream discharge trigger changes in sediment, morphology, and vegetation. The mass of water, for example, supplies sustenance for vegetation, and the energy that water expends as it descends through a channel contributes to erosion, transport, and redeposition of sediment. The energy expended by water flowing downgradient through the river performs geomorphologic work. It rearranges sediment, building such landforms as islands and bars attached to the banks and erodes previously existing forms. The significance of those features is that they are the foundations of vegetation and species habitats.

Sediment, in turn, influences the morphologic characteristics of the channel. As outlined in Chapter 2, the central and lower Platte River have a braided channel, but the variety of forms is shaped by interactions between the volume of sediment to be transported and the water, with its accompanying energy, that is available for the work of transport. When the available energy is less than that required to transport the available sediment, deposition occurs, bars develop, islands expand, and beaches encroach into the channel area. When the available energy is sufficient to entrain and transport the available sediment that is temporarily stored in bars, islands, and beaches, erosion results, and these features become smaller.

Riparian forests are influenced by abiotic ecological processes, such as the dynamics of water, sediment, and morphology of the river. Surfaces created by deposition are potential seedbeds for young plants. In the absence of fire and grazing, if flows do not sweep away or bury new plants and their substrate, the vegetation is likely to become relatively permanent and be in the form of a cottonwood and willow woodland. The resulting vegetation then has a feedback effect on the physical processes: the trees introduce hydraulic roughness to the channel and its nearby surfaces and thus induce additional deposition, which leads to higher floodplain surfaces that further influence vegetation. Under low natural flows, a broad, braided channel (with many subchannels) may be converted to a narrower system of fewer channels. That chain of events also can be reversed by higher natural flows, or clearing of vegetation, which can expose surfaces to energetic flows that might entrain sediment and erode the surface.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

Those hydrological connections are particularly relevant for threatened and endangered species. DOI’s perspective on the connections is outlined by the U.S. Fish and Wildlife Service (USFWS, unpublished material, June 16, 2000). Whooping cranes prefer roost sites that include shallow water bars that are surrounded by deeper channels and that have long sight lines (unvegetated areas)—a set of conditions common along the Platte River of a century ago. Piping plovers and interior least terns prefer a habitat that has unvegetated areas with sandy surfaces exposed by receding river flows during the breeding season of late spring. If nests are to produce fledglings, they cannot be inundated by pulse flows. Pallid sturgeon appear to prefer streams with sandy bottoms for foraging and a series of annual flows that have natural fluctuations, with high spring flows and lower flows in late summer (USFWS, unpublished material, June 16, 2000).

The causal chain of adjustments in the Platte River ecosystem begins with alterations of ecological processes, such as hydrology, and perhaps fire, grazing, and invasive species. Flows respond to two primary influences: climate and human regulation. Climatic influences through variation in the rainfall and snowpack that supplies river discharge change on time scales of a decade or more. Because much of the water flowing through the central and lower Platte River originates as precipitation over the Rocky Mountains, climatic changes in the mountainous areas are more relevant to flow in the central and lower Platte River than are climatic changes in Nebraska.

Human influences on Platte River hydrology are long term and short term. Large storage dams in the upper reaches of the Platte River Basin through the 1900s changed river discharges by lowering flood peaks and by releasing more water during summer months than was released during the summers before the dams were built. Other major changes include the installation of diversion works on the river system that divert some or all of the flow during portions of the year. Water often returns to the channel from return canals or groundwater seepage. The short-term effects of human regulation of the river include management of water delivery from storage sites to downstream water-users, adjustments in flow to generate hydropower during periods of high demand for electricity, and scheduling of return flows. Groundwater, which is influenced by pumping and seepage from fields and unlined canals, also contributes to stream flow.

Human-induced changes in the controlling flows of water in the Platte River Basin are larger and more important than climate-induced changes in controlling the Platte River ecosystem and habitats for endangered species. The storage reservoirs in the Platte River Basin store about 6 million acre-ft of water, and they have reduced flood peaks by 80% or more in the central Platte River (Figure 2-6). Those changes are at least half an order of magnitude greater than any changes envisioned for river systems as a result

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

of climatic change from global warming (Arnell 1996). Simpson (2003) observed a similarly important role for human activities in changing channel conditions on the South Platte River in Colorado. Although storage reservoirs, diversions, power houses, and return flows have apparently caused changes in habitats along the river, these same engineered features offer an opportunity for ecosystem restoration. If the water-control infrastructure can be operated in such a manner as to partly mimic the natural flows that once dominated the river and if other ecological processes can be re-established, it is likely that habitats will respond by reverting to conditions that are more like those preferred by the listed species (Murphy and Randle 2001).

Given the magnitude of engineered changes and the economic value derived from them (such as flood control and water supply for agricultural or urban uses), it is not possible to completely restore predevelopment flows and the habitats they created. Bison will never again mass along the shores of the Platte. Even so, it is possible to partially recreate the presettlement conditions, given appropriate knowledge about the connections among the various subsystems that make up the Platte River ecosystem. In geographic locations where more extensive restoration is not possible, some management actions (such as wetland creation) may be necessary to complement restoration activities. In general, because habitat requirements of individual endangered species that use the central Platte are linked to more natural functions of the entire ecosystem, it is possible to pursue ecosystem restoration goals even when the presettlement conditions cannot be fully attained. In contrast with ecosystem restoration, simply combining habitat-management goals for individual species quickly leads to conflicting management options that may be mutually exclusive, especially when other species are affected. Management of songbird habitat as opposed to crane habitat is one contentious example that was illustrated in presentations to the committee. Compared with individual-species management, ecosystem-restoration approaches tend to be more stable, will include more (but not all) species successfully, and provide a larger geographic and temporal scale on which work can be accomplished. Restoration of conditions more similar to presettlement conditions than the present arrangements will be likely to benefit a wide range of species, including valuable waterfowl. Such restoration is also likely to result in more stable conditions and populations.

If the hydrological regime is adjusted to become more like the original, natural regime with the influence of dams, native species are likely to be favored because they established communities in a river regime without dams. Often, regimes that are highly unnatural eliminate the advantages of native species, and invasive species are more successful.

DOI research and investigations by others supported two types of management decisions targeted to maintain habitat and enhance benefits to the

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

listed species. First, DOI identified a series of operating rules for the flow of water in the river; these rules became the agency’s recommendations for instream flows. Recommended instream flows are important because they represent DOI’s vision of the kind of river hydrology that most directly affects habitat of individual species, particularly the movement of sediment, the forming of the channel, and periodic high or low flows. According to the DOI view, instream flows can benefit the threatened and endangered species by hydrologically creating or maintaining preferred habitat conditions. Second, DOI identified connections among water, sediment, morphology, and vegetation that predict how the Platte system may respond to recommended instream flows. Besides mechanical removal of vegetation and some wetland manipulations, DOI did not appear to consider restoring any ecological processes.

Available Data on the Platte River Ecosystem

One purpose of this report is to evaluate the scientific validity of DOI’s conclusions about what the instream flows should be and how they influence other aspects of the Platte River ecosystem. Part of that evaluation entails a review of the available data that DOI used to reach its conclusions. The following paragraphs assess the data available to DOI for its instream-flow and ecological research. Assessments of the data pertaining to the threatened and endangered species are presented in later chapters, where each species is considered in detail.

The most important data used as input for explanations and predictions of Platte River habitat needs of the listed species are related to water, sediment, channel morphology, and riparian vegetation. The water-discharge data on the Platte River are from a set of gaging stations that have varied lengths of record (Figure 4-1). The gages with the longest records in the central and lower Platte River are on the mainstem Platte River above the Loup River confluence and near Duncan, Nebraska, where measurements began in 1890. Both records are discontinuous, so their value for analysis and modeling is diminished. The longest continuous record (1925+) for the central and lower Platte River is from gages on the river near Overton, Duncan (Figure 4-2), and Ashland, Nebraska. Data from a nearby location at Lexington extends the Overton gage record back an additional decade. As the twentieth century progressed, the U.S. Geological Survey (USGS) installed additional gages on the river; by 1955, there were six recording sites in the central and lower Platte River, three on local tributaries, and six more in the upstream reaches of the North and South Platte Rivers.

Examination of the stream gage data for the central and lower Platte River shows that before 1925 measurements were sporadic and accom-

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 4-1 Periods of stream-flow record from gaging stations in Platte River Basin. (1) Includes four sites: Saratoga, WY; upstream of Seminoe Dam; upstream of Pathfinder Dam (historical); and downstream of current Pathfinder Dam site (historical). (2) Includes data from records for Camp Clarke (1896-1900), Mitchell (1901; 1907-1911), Scottsbluff (1912), Oshkosh (1929-1930), and Bridgeport (1902-1906; 1915-1928; 1931-1998). (3) Includes data from historical record for Platte River near Lexington, NE, before 1925. (4) Includes data from records for Platte River near Central City (1922-1927) and Columbus (1895-1914; June 15-Oct. 31, 1928). (5) Sum of Platte River at Columbus, Loup River near Genoa, and Loup River Power Canal. (6) Consists partly (Oct. 1, 1960-July 2, 1988) of record synthesized by combining Platte River at Louisville with Salt Creek near Ashland and Platte River at North Bend with Elkhorn River at Waterloo. (7) Includes data from record for Loup River at Columbus, NE, before Oct. 11, 1978, with Loup River Power Canal diversion added beginning Jan. 1, 1937. (8) Includes data from historical record for Elkhorn River at Arlington, NE. (9) Consists partly (Sept. 30, 1969-Dec. 31, 1998) of record synthesized by regression based on Salt Creek at Greenwood, NE. Source: Adapted from Stroup et al. 2001.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

plished with techniques that would be considered unreliable today. In addition, the Platte River is a difficult stream to gage because its channel banks and bed are unstable; this complicates the calculations necessary to produce a stream-discharge measurement. That situation comes about because early calculations involved only two variables: channel width and depth of flow. Such an approach was incapable of taking into account the rapid changes in channel geometry that are common in sand-bed braided rivers. Methods became more sophisticated and discharge measurements more reliable after about 1925. The post-1925 stream-gage record is useful for supporting explanations of channel dynamics and interactions with sediment and vegetation, and the record documents short-term changes of a few years in duration. DOI analysts can specify with confidence the process connections between stream-flow and sediment transport, riparian vegetation, and inundation of various surfaces in and near the channel. Long-term changes in discharge are still difficult to specify, however. In searches of the stream-gage record for cyclic changes in discharge, for example, a common rule of thumb for hydrological analysis is to require a record that has 2-4 times the length of the suspected cycle; in this way, the cycle is repeated often enough in the record to be confidently identified. Hydrological adjustments to climatic changes occurring in cycles of several decades to a century, therefore, are not yet identifiable in the stream-flow record for the Platte River. The additional complications of variations in exchanges between surface water and groundwater are also difficult to discern in the stream-flow record.

The long-term gaging data from the North Platte, South Platte, and Platte Rivers show that river discharge has changed (Murphy and Randle

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

FIGURE 4-2 Daily stream-flow record for Platte River near Duncan, Nebraska, showing length of record and variability of flow. Source: USGS 2003.

2003). The flow record can be divided into four periods, separated by the gaging record and installation of engineering structures in the system: 1895-1909, 1910-1935, 1936-1969, and 1970-1999. There was a steady decline in the mean daily discharge of the rivers at a variety of measurement points from 1895 to 1969. Since 1969, however, flows have increased (Table 4-1), probably because of transmountain diversions into the river from outside the basin and return flows from high groundwater. The increased annual discharges through the Platte River in the later parts of the record have not

TABLE 4-1 Annual Mean Platte River Flows, cubic feet per second

Gaging Station

1895-1909

1910-1935

1936-1969

1970-1999

North Platte River at North Platte, NE

3,190

2,750

646

862

South Platte River at North Platte, NE

582

492

322

619

Platte River at North Platte, NE

3,780

3,240

968

1,480

Platte River near Cozad, NE

3,550

3,020

461

981

Platte River near Overton, NE

3,660

3,160

1,140

2,100

Platte River near Grand Island, NE

3,580

2,950

1,080

2,110

 

Source: Randle and Samad 2003.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

had appreciable effects on the system, because the system adjusts its geomorphology only during high-discharge (flood) events. The increased annual discharges are passing through the system as longer-duration, relatively low-conveyance flows.

The magnitude of the annual flood has followed a course of change similar to the changes in annual discharges, but the changes in annual floods have been smaller (Table 4-2). As a result, annual floods during the period 1970-1999 are not nearly as large as those in the system before 1909. In the extreme case, at the gage site on the Platte River at Cozad, the magnitude of the recent annual flood peaks is only 15% of the magnitude observed before 1909. The large upstream dams were built to exert this flood control and to save the water from the flood peaks in reservoirs for later, slower releases for irrigation and urban water supply. The significance of the earlier and larger annual floods is that they performed most of the geomorphic work of changing the channel configuration, maintaining a wide active channel, adding and deleting islands and bars, and sweeping seedlings from exposed sand accumulations. Those functions have not been performed by the recent and smaller floods.

To investigate the flow movement and distribution in the entire basin, a central Platte OPSTUDY hydrological model has been developed as a management tool to incorporate all hydraulic structures, water-rights demands, and so on. The basic function of this mass-balanced hydrological model is to simulate all the flow inputs and outputs at various locations from the inflows of the North and South Platte River down to the flow at the lower Platte River. Reservoir flow storage, flow diversions, irrigation demands, and groundwater inputs and outputs are included in the model.

The earliest descriptions of the vegetation and morphology of the channel appear in journals and accounts by explorers beginning in the early 1800s. Qualitative assessments described a river with scattered trees on its

TABLE 4-2 Annual Flood Peaks in Platte River Basin (Flows with Return Interval of 1.5 Years), cubic feet per second

Gaging Station

1895-1909

1910-1935

1936-1969

1970-1999

North Platte River at North Platte, NE

16,300

8,150

2,160

2,380

South Platte River at North Platte, NE

2,330

1,430

712

1,420

Platte River near Cozad, NE

17,600

9,140

1,980

2,590

Platte River near Overton, NE

19,400

9,000

3,490

4,750

Platte River near Grand Island, NE

17,300

10,100

4,500

6,010

 

Source: Randle and Samad 2003.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

banks and immense numbers of wooded islands all coursing through an open prairie landscape (Johnson and Boettcher 2000a). Explorers occasionally reported quantitative estimates of channel dimensions. Channel depths were estimated at 2-4 ft, and bank-to-bank widths were up to 2 mi in some places (Mattes 1969). Starting in the late 1850s, the GLO conducted surveys to implement the Township and Range Survey system that is the basis of all modern property descriptions in the Midwest. The surveyors’ notes and plat maps derived from them included channel characteristics, descriptions of large islands, general vegetation, size and species of witness trees, soils, geology, human uses of the land, and depths of flow in the river. The GLO surveys generally provide the first systematic record of the predevelopment environment of the central and lower Platte River. With the construction of the Union Pacific Railroad, photographers began recording images of the river (Figure 4-3).

The advent of modern surveys of river cross sections was associated with bridge construction from the late 1800s on. The data are probably accurate, but they were limited to crossing sites. Bridge cross sections do not necessarily reflect the kinds of channel dynamics that occur in more-natural reaches inasmuch as they tend to be narrower and more stable than other cross sections. The first complete coverage of the geomorphology of the river resulted from the USGS topographic mapping program, which produced maps for Nebraska and the river based on conditions in 1896-1902. Private maps followed, showing parts of the river at various dates

FIGURE 4-3 Photograph of central Platte River near 100th Meridian in vicinity of Cozad, Nebraska, in 1866. River has wide active channel mostly devoid of vegetation except for few trees, apparently cottonwood, on island. Source: Carbutt 1866b. Reprinted by permission of Union Pacific Historical Collection.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

and stages. Channel structure and vegetation were systematically photographed from the air for the first time in 1938. Later aerial photography and other remote-sensing imagery, including infrared imagery, provide views of the system at roughly decade intervals to the present.

Cross-sectional surveys of the river as input data for hydraulic modeling date from about 1980. Eschner (1983) surveyed six cross sections across the central Platte River during 1979-1980 and related changes in channel geometry to changes in discharge. Thereafter, DOI researchers surveyed various cross sections at various times, and other state agencies and individual researchers surveyed additional cross sections. There is no central repository of cross-sectional data; for many river locations, only one set of cross-sectional data exists. For reaches with multiple surveys, DOI investigators have produced quantitative information about channel change. Researchers documented these repeat survey sites (Randle and Murphy 2003), and they now provide a benchmark for monitoring and input for modeling.

Sediment-discharge measurements are not available for the central and lower Platte River, so investigators have used model calculations to estimate sediment transport through the system. Input for sediment-transport models includes daily water discharge and estimated (or assumed) amounts of sediment discharge. Such fluvial geomorphology and hydrological engineering models are numerous (Simons and Senturk 1992). The initial model output is in the form of mathematical curves that generalize the connection between water and sediment discharge. Further calculations use the sediment rating curves and gaged water discharges to produce the estimates of total sediment transport. The best understanding of sediment discharge through the central Platte River derives from DOI work summarized by Randle and Samad (2003), who used three sediment-transport functions to make connections between water and sediment discharge, models by Kirchner (1983) and Simons and Associates (2000), and a model constructed by Randle and Samad. Although the exact numbers for results varied from one model to another, sediment trends were highly similar in all models. High correlation among the models was somewhat expected in that the major variable in the calculations was water discharge, and the same values were used in all the calculations.

The results of sediment-transport and discharge studies in the Platte River show that the roles of the North and South Platte Rivers as sources of sediment for the Platte River in Nebraska have changed. From 1895 to 1935, the North Platte supplied much more sand to the Platte than did the South Platte (Murphy and Randle 2003); but from 1936 to 1999, the South Platte supplied at least as much as the North Platte. Patterns of sediment transport through the Platte River also changed. From 1895 to 1935, volume and rate of sediment transport were consistent along the entire length of the central Platte; but after 1936, the loads from Cozad were lower than

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

the amounts transported past the town of North Platte. The decrease was probably the product of sediment deposition in the 70-mi reach from North Platte to Cozad, which is also the location of the Johnson-2 (also known as J-2) return flow (Figure 4-4). Also after 1936, the amount of sediment transported for reaches downstream of Cozad and the Johnson-2 return flow were higher than the amount at Cozad. The increase was probably the result of erosion in upstream reaches.

The central Platte River has two distinct segments from the perspective of sediment: the segment upstream of Cozad and the Johnson-2 return, which is accumulating sediment (aggrading), and the segment downstream of Cozad and the Johnson-2 return, which is losing sediment (degrading). That circumstance is likely to lead to different geomorphic forms in the two reaches. As might be expected, the distribution of sediment transport over time and through space is explained mostly by changes in stream flow, largely as a result of controls by dams, diversions, and return flows. All hydrological and geomorphic processes operate amid cyclic weather patterns typical of western extremes and more gradual, longer-term climatic changes.

Information on riparian vegetation has many of the same sources as geomorphic data. Early descriptive accounts from explorers and settlers provide qualitative information and were followed by more quantitative data from the GLO survey, aerial photography, and finally satellite imagery. Johnson and Boettcher (2000a) reviewed the journals and diaries of explorers and surveyors from the early 1800s to the 1870s. They concluded that scattered timber was found along the outer banks, which were susceptible to prairie fires, while heavier woodland occurred on numerous islands of all sizes in the channel. Surveyors recorded the species and size of hundreds of living witness trees but also found the stumps of many trees, indicating extensive deforestation in progress by settlers, soldiers, and railroad crews. Little quantitative information is available on the Platte’s woodland cover between the 1870s and the first available aerial photographs in the late 1930s. Johnson and Boettcher (2000a) suggested that cutting of trees by prairie farmers continued for decades after settlement and contributed to the open appearance of the river in the early photographs. Currier and Davis (2000) questioned whether such clearing took place, but U.S. Surveyor Amherst Barber noted in 1899: “Bearing trees were not used at aforesaid corners for the reason that the whole of the river bed and islands is constantly searched by farmers from the prairies, who cut and carry off all poles large enough to cut, and the small sapling I found, of 2 in. or so, would soon be removed.”

The foregoing discussion about woodland occurs in the following important context. Woodland probably has always existed along the central and lower Platte River during the past two centuries, but it has occurred in

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-4 Central Platte River with associated water control infrastructure and places of interest mentioned in this report. (1) Lake McConaughy. (2) Kingsley hydroelectric plant. (3) Lake Ogallala. (4) Keystone/Sutherland Canal. (5) River channel Kingsley to North Platte. (6) Sutherland Reservoir/Gerald Gentleman Station. (7) Lake Maloney/North Platte hydroelectric plants. (8) Central diversion. (9) Jeffrey return. (10) Johnson Lake/Johnson-1, Johnson-2 hydroelectric plants. (11) Johnson-2 return. (12) encroached reach (river channel below Central diversion to Johnson-2 return). (13) Cottonwood Ranch. (14) Kearney Canal. (15) Audubon Rowe Sanctuary. (16) Platte River Trust lands. (17) Korty diversion. (18) Korty Canal. (19) Kearney bridge. Source: Adapted from USFWS, unpublished, June 5, 2000.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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different amounts. By the mid- to late 1800s, it may have been at a minimum, but at the very least it occupied mid-channel islands. In the twentieth century, woodland had expanded and covered more areas than in the late 1800s. In all cases, however, there has been a gradient of environments ranging from quite open situations to closed woodland. More specifically, woodland cover apparently was minimal during the late 1800s and early 1900s. Comparison of the woodland distribution in the riparian zone from 1938 (when aerial photography of the region began) to present times shows extensive woodland expansion along many reaches of the central Platte River (e.g., Williams 1978; Eschner 1983; Johnson 1997). Those long-term records indicate that woodland cover expanded and reached a maximum in many reaches of the river beginning in the 1960s. Since the 1980s, managers have cleared substantial areas of woodland for habitat-management purposes, but other change has been minor.

The riparian vegetation associated with the Platte River has changed dramatically in the last 150 years. The extensive island woodland was cleared for timber by the late 1800s. Woodland expanded sharply in the first half of the 1900s in connection with the construction of dams and diversions that caused an increase in the recruitment and a decrease in the mortality of riparian woody plants. By the 1960s, equilibrium among flows, sediments, land form, and woodland appeared in most reaches of the river (Johnson 2000).

Scientific Data for Ecosystem Connections

Various datasets on the corridor of the central and lower Platte River support systematic understanding of connections among the major ecosystem components: water discharge, sediment movement, morphology of the river, and riparian vegetation. (Other ecosystem components, including grazing and invasive species, have not been analyzed.) Collectively, those characteristics determine the nature of habitat available for wildlife in general and of suitable habitat for threatened and endangered species in particular. DOI agencies accomplished three tasks in the process of listing the four protected species and in designating critical habitat for the whooping crane and piping plover. First, DOI developed an understanding of the species habitat needs and then an understanding of the river system dynamics. Second, DOI determined the magnitudes, frequency, duration, and timing of flows in the river to benefit the listed species. Finally, DOI recommended a set of instream flows to benefit the listed species. This section addresses river ecosystem and habitat interconnections, the next section reviews instream flow issues, and a third section addresses the habitat suitability issue.

Data outlined earlier in this chapter paint a general picture of environmental change in the corridor of the central and lower Platte River. The relative abundance of habitat conditions favorable to the listed species

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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seems to have declined over the last century. The alteration of flows by human controls has substantially changed the behavior of the river. Human controls have reduced flood peaks, eliminated many pulse flows, and decreased the total amount of water flowing through the system. The decline of discharges since 1895 has reversed in recent years, but not to the point of recovery to predevelopment levels.

The water-control infrastructure and related water-flow changes have larger effects throughout the river’s subsystems. Flow diversions effectively dewater the river in some reaches, resulting in the deposition of sediment, particularly upstream of the Johnson-2 return (Figure 4-5). Flow in the lower reaches is sufficient to move sediment, so erosion proceeds there because sediment influx from upper reaches is interrupted (Figure 4-6). Reductions in flow events, particularly those about bankfull (a water discharge that fills the channel cross section but does not spill over onto the floodplain), and changes in the sediment transport regime are forces that play themselves out on a geomorphologic stage. The characteristic forms that once dominated the river—a braided configuration laced with intertwined channels, bars, islands, and beaches of great complexity—have become simplified. Features that once would have been unvegetated and mobile have become vegetated and stabilized in the central Platte River (Figures 4-7 and 4-8).

Extensive reports document generalizations about the dynamics of water, sediment, morphology, and riparian vegetation. Summary explanations, supported by numerous references, of the physical changes in water, sediment, and morphology of the river appear in Nadler and Schumm (1981), Simons and Simons (1994), and Murphy and Randle (2003). The changes

FIGURE 4-5 Johnson-2 diversion on central Platte River, a feature that influences local distribution of flows in river and provides valuable irrigation water for agriculture. Source: Photograph by W.L. Graf, May 2003.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-6 Aerial photographs from 1938 (left) and 1998 (right) of Platte River at Johnson-2 irrigation return site (for location, see site 11 south of Lexington on Figure 4-4). Irrigation return drain is on left side of each image. In 1938 view, channel above return is open and sandy with wooded islands; in 1998 view, channel is covered with vegetation except for relatively narrow active channel and clearer area downstream of return point. Channel is bifurcated in this reach. Source: USBR 1998.

associated with riparian vegetation and connected physical changes have been the focus of some debate, but clear understanding has emerged (Johnson 1994, 1997; Johnson and Boettcher 2000a; Currier and Davis 2000). Currier (1995) has explained further connections with groundwater. Many studies of the channel history of the Platte River have been conducted. Most time-series graphs start with channel widths measured on GLO plat maps, which are followed by estimates made from aerial photographs, first available in the late 1930s. Standard methods have not been used or developed to measure width or area, so comparisons among studies are difficult; studies vary widely in reaches sampled, dates of photography, sample sizes (number of transects or sample points), maximal vegetation allowable to qualify as “open” or “unvegetated” channel, and use of error analysis.

Eschner’s (1983) oft-cited graphs show a steep decline in channel width between the time of the plat maps and the first aerial photographs (Figure 4-9). The decline would be less steep if nonsurveyed vegetated islands could

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-7 Aerial photographs from 1938 (left) and 1998 (right) of Platte River at western edge of Cottonwood Ranch (for location, see site 13 south of Kearney on Figure 4-4). During 60-year period between two dates, woodland has expanded to cover most of river area. Source: USBR 1998.

FIGURE 4-8 Aerial photographs from 1938 (left) and 1998 (right) of Platte River immediately downstream of Kearney bridge crossing (see Figure 4-4 for location). In 1938, abutments of bridge and approach berms for road constricted channel, leading to narrowing at western edge of 1938 image. At that time, channel was devoid of vegetation except for forested island in center of image. In 1998, bridge and its approaches were removed, and channel was largely vegetated except for narrow, active portions. Source: USBR 1998.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-9 Changes in channel width at various cross sections of Platte River as interpreted by T. R. Eschner on basis of GLO plat maps (1860s) and aerial photographs (1938 and later). Greatest amount of narrowing has occurred in upstream or western cross sections, with decreasing amounts of change in downstream or eastward direction. Source: Eschner 1983.

have been accounted for in the width calculations on the plat maps. Johnson’s channel-area estimates (1994, 1997) did not begin with the plat maps, because of uncertainties in accounting for the missing islands. Rather, they began in 1938 when aerial photographs first became available; that is, any woodland expansion before 1938 was not estimated. Several quantitative patterns since 1938 are evident (Figure 4-10). Channel-area decline has been more extreme in upstream reaches of the Platte River; this suggests downstream attenuation in the effects of flow regulation. Channel-area decline occurred earlier in upstream than in downstream reaches. Channel area in many reaches stabilized or began to increase in the 1960s, so equilibrium or steady-state conditions probably began nearly 4 decades ago. Declines in channel width and area were caused by vegetation colonizing and surviving in the active channel. The vegetation varies considerably in

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-10 Changes in channel width at various cross sections of the central Platte River as interpreted by C. Johnson on basis of aerial photography. In later part of record, after 1960s, decreases in width remained stable or reversed. Source: Johnson 1997. Reprinted with permission; copyright 1997, John Wiley & Sons Limited.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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composition and in structure and is highly dynamic in time and space (Currier 1982; Johnson 1994). The initial composition of vegetation on new sandbars depends on the season of drawdown and exposure. Generally, exposure early in the growing season (May and June) allows for colonization by perennial, pioneer woody plants (cottonwood, Populus; and willow, Salix) and many annual plants (such as Echinochloa, Eragrostis, Xanthium, and Cyperus), whereas first exposure late in the growing season (August) produces domination almost exclusively by annuals. Sandbars with only annuals convert back to unvegetated surfaces the next year. Sandbars with young perennial vegetation (especially if cottonwood and willow seedlings are present) will continue to develop more-complex and more-diverse vegetation unless it is killed by erosion or sedimentation. Woody vegetation on sandbars that has survived 3-5 years is relatively resistant to both processes.

What caused the channel narrowing and woodland expansion? Several single-factor explanations have been offered, including a decline in peak flows (USFWS 1981a) and increases in summer low flows (Nadler and Schumm 1981). The most recent published analyses (Johnson 1994, 1997, 2000) support the following multifactor explanation. Reduced stream flow in late spring (the period when reservoirs are filled) during the peak seed-dispersal period for pioneer cottonwood and willow trees best correlates with the historical rates of woodland expansion. Lower flows during this period after regulation enabled more extensive reproduction of trees on the exposed sandbars than in the predevelopment period. Reduced flows at other periods tended to increase tree-seedling survival and thus contributed to the expansion of cottonwood and willow woodland into formerly active, unvegetated portions of the channel.

Survival of tree seedlings also may have been increased by higher late-summer (irrigation-return) flows in the regulated river; however, the GLO notes indicate that the Platte River rarely went dry, contrary to popular belief based on some settlers’ diaries (see Edmonds 2001 regarding pitfalls of written records). Additional causes of modification of local conditions in some areas are grazing and invasive species.

Three invasive riparian plant species are of most concern to Platte River managers: a Eurasian exotic, purple loosestrife (Lythrum salicaria) (Figure 4-11) and invasive nonnative variants of two grasses—common reed (Phragmites australis) and reed canary grass (Phalaris arundinacea). Lythrum and Phragmites are known to propagate and spread from root and stem fragments, so the practice of mowing and disking infested Platte River sandbars may be causing proliferation.

The relationship between open channel and wooded areas is dynamic. Essentially, the modern Platte River is a microcosm of its earlier, predevelopment form but operating under modified ecological processes.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-11 Purple loosestrife, an introduced, nonnative species that aggressively occupies some niches along Platte River. Source: Photograph by W.L. Graf, August 2004.

When stream flow was reduced, the river was less able to rework or otherwise influence all its formerly active floodplain. About half the floodplain of the central Platte River reverted to woodland that became relatively stable in area and in location; however, bank erosion along active channels in these wooded areas creates new areas of open channel incrementally (Figure 4-12). The remaining relatively unwooded area of the floodplain that comprises open channel and young vegetated sandbars is, conversely, highly dynamic. Vegetation comes and goes, depending on flow conditions. For example, from 1986 to 1995, considerable areas of former channel became vegetated, and former vegetation became channel (Figure 4-13). Although there was considerable turnover and relocation of channel and vegetated categories, net channel area was generally constant. Channel area increases during periods of high stream flow and decreases during periods of low stream flow.

The present conditions have resulted from increased width and depth in the remaining active channel as other minor channels were deactivated by the growth of new woodland. Instead of being dispersed among several

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-12 Aerial photographs from 1938 (left) and 1998 (right) of Platte River at Audubon Rowe Sanctuary (for location, see site 15 southeast of Kearney on Figure 4-4). Channel in this reach is in two parts separated by large island. In 1938, both parts were open and mostly without vegetation except for small islands that were wooded. In 1998, active channels are more narrow, but southern channel remains braided. Source: USBR 1998.

subchannels, a single channel carries most of the flow in some reaches (Figure 4-14). That process has reduced the recruitment and increased the mortality of tree-seedling populations. Mortality of tree seedlings is extremely high in the modern river. Johnson (2000) found that on about 90% of the new sandbars all seedlings died by the end of their first year of life. The flow now fits the remaining channel better, and summer stream-flow pulses from thunderstorms and unused water deliveries during wet weather and moving ice in winter have become increasingly effective in killing young tree seedlings by erosion and sedimentation. Summer desiccation is another mortality factor, but it occurs at a much lower frequency than mortality due to flow and ice.

Habitat implications of the newly established woodland cover are complex. In reaches where the total distribution of woodland has no net change in coverage, the remaining open area may no longer have sufficient size or channel structure to support species that require open and wide channels. Increased woodland cover has caused increases in other species, such as deer, turkeys, and songbirds.

Some reaches may not have reached equilibrium or may have disequilibrated. Currier (1997) reported that channel narrowing and vegetation expansion had occurred between the middle 1980s and the middle 1990s in several reaches not examined by Johnson (1994). Johnson (1997) and Currier (1997) both measured rapid vegetation expansion in a rela-

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-13 Map of areas of habitat change in cover from 1986 to 1995 for reach of Platte River near Shelton, Nebraska. Source: Johnson 1997. Reprinted with permission; copyright 1997, John Wiley & Sons Limited.

tively wide channel near Grand Island during the same period but interpreted the cause differently. Johnson (1997) speculated that release of sediment after the clearing and leveling of islands upstream of Grand Island had aggraded the riverbed, altering the flow splits among channels, dewatering the main channel, and causing the vegetation expansion. Currier (1997) speculated that the cause of the narrowing at Grand Island was alteration of stream flow by upstream water development. Resolution of the equilibrium issue was difficult because the authors used different methods, reach boundaries, and aerial photographs.

Most of the Platte’s woodlands now have a mature canopy of cottonwood and willow, with an understory of elm, ash, and cedar. As a result of systemic controls that include natural and human influences, there is little regeneration of cottonwood and willow, and the mean age of the existing woodland is increasing in the absence of ingrowth. Cottonwood and willow will decline in importance over time, with corresponding declines in biodiversity, unless measures are taken to promote their regeneration and establishment. There is also

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-14 Aerial photographs from 1938 (left) and 1998 (right) of Platte River west of railroad bridge near Gibbon (see Figure 4-4 for location). Between 1938 and 1998, channel converted from braided open channel to modified version with braided and single-thread components side by side, with woodland covering portion of 1938 active channel area on north side. Source: USBR 1998.

some variety of densities, with some open areas and some areas with gradational densities between open and closed woodland.

Despite varied interpretations, studies of woodland processes in the central Platte River generally agree on the following. First, woodland was an important and permanent part of the presettlement Platte, but its aerial coverage has increased substantially in the modern, regulated river. Second, woodland expanded in many areas from the 1930s to the 1960s. Third, a relatively stable relationship between the areas of woodland and open-channel area has developed in most reaches since the 1960s, with stability of composition and structure. Fourth, some reaches continue to exhibit channel narrowing, but the cause of the narrowing in these reaches has not been completely explained. Fifth, the different conclusions reached by investigators on these issues have been difficult to resolve because of differences in study areas and methods.

A MODEL FOR RIVER AND HABITAT CHANGE

General explanations provide the basis of a more exacting analysis of the interactions among water, sediment, form, and vegetation. USFWS used the Physical Habitat Simulation System (PHABSIM) to make connections among the various components of the ecosystem of the river. PHABSIM is a collection of interactive programs that accept as input such hydrological measures as water depth, velocity of flow, substrate, and cover (Figure 4-15). The program

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-15 Schematic diagram of basic operations of Physical Habitat Simulation System (PHABSIM) used by DOI agencies to specify connections among habitat characteristics, habitat preferences, and river discharges. Abbreviations: WSL, Water Surface Elevation. IFG4, STGQ, MANSQ, WSP, VELSIM, AVDEPTH/AVPERM, HABTAE, HABEF, HABTAM, are models and programs. Further information and definitions available at http://www.fort.usgs.gov/products/Publications/15000/appendix.html. Source: Waddle 2001.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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takes into account coincidental data about the amount of suitable habitat for listed species in and along the river at a variety of discharges, and it builds a series of mathematical relationships between the magnitude of flow and the amount of usable habitat per unit length of the stream. The general mathematical relationship can be expressed graphically (Figure 4-16). There may be many such curves for each species of interest, with each curve representing the amount of suitable habitat available during different life stages of the species.

PHABSIM assesses the habitat value of a reach of stream over a range of discharge for a target species or life stage (Bovee et al. 1998). The model has seen service in a wide variety of environments and for many species and has even been used to assess recreation potential and aesthetics (Gillilan and Brown 1997). In most applications of PHABSIM, patches of habitat are characterized at different rates of discharge in terms of three “microhabitat” variables: depth, water velocity, and substrate size. Substrate size in each patch is estimated from field data, and depth and velocity are estimated by a spatially explicit hydraulic model; both one- and two-dimensional models have been used. At each discharge, the habitat value of each patch is evaluated independently for each variable according to “suitability curves” that range from 0 to 1, and the product of these values and the area of the patch

FIGURE 4-16 Schematic example output of PHABSIM. Graph represents amount of habitat for particular species that is available at each flow discharge of river. System generates one such graph for each life stage of species of interest. At some flow discharge, amount of available habitat is at maximum peak of curve. Source: Adapted from Gillilan and Brown 1997.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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yields an estimate of weighted usable area (WUA). These are summed over patches, producing a curve of WUA over discharge for the reach.

Although PHABSIM is widely used, it has been challenged on the basis of both its approach and its application (Castleberry et al. 1996). It has some limitations. The one-dimensional hydraulic models use dubious methods to estimate the variation in velocity across the channel, and the typical spatial scale of the two-dimensional models does not match that of the suitability curves (Kondolf et al. 2000). The accuracy of the velocity estimates is seldom tested, and reported tests show weaknesses in results (Kondolf et al. 2000). The biological meaning of WUA is unclear: Bovee et al. (1998, p. 71) say only that it is a “microhabitat metric.” However, the modeling implicitly assumes that habitat preferences do not vary with physical and biological conditions in the stream and that density is a valid indicator of quality; both these assumptions are dubious (Bult et al. 1999; Holm et al. 2001; van Horne 1983). Applications of PHABSIM often do not report confidence intervals for estimates of WUA (Castleberry et al. 1996). Those reservations aside, PHABSIM has been the standard for habitat assessment along rivers. The U.S. Bureau of Reclamation (USBR) is developing an alternative sediment-vegetation model, SEDVEG, to evaluate the interactions among hydrology, river hydraulics, sediment transport, and vegetation for the Platte River, but it has not been widely used.

The application of PHABSIM to the Platte River beginning with investigations in the late 1980s and the early 1990s produces a valid starting point for assessing the complex web of connections among water flows, sediment processes, geomorphology, and habitats. There remains considerable uncertainty regarding exact specifications of particular flow discharges to accomplish habitat restoration because of the complexity of the channel system and the lack of substantial empirical studies to connect the fluvial processes with the resulting habitat. In many cases, judgments had to be made about the most likely approaches to successful restoration of the river processes. Uncertainty also enters the analysis because it is unclear that if habitat is reconstructed, it will serve the needs of the threatened and endangered species. However, as discussed in Chapter 3, science does not offer perfect certainty. Rather, it offers reasonable approaches based on observable experiences. The application of PHABSIM and later improved strategies, such as the establishment of normative flows, are steps in a process of improving understanding of the habitat needs for the species rather than sharply defined end points.

Instream-Flow Recommendations

Once analysts define the connections among various measures of flows and habitat for species of interest with the PHABSIM model, the calcula-

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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tion of flow recommendations with a second family of models collectively known as Instream Flow Incremental Methodology (IFIM) is possible. IFIM includes the label “incremental” because the system of equations and data can simulate the habitat implications of incremental changes in flow. IFIM uses computer software to evaluate microhabitat and macrohabitat characteristics that occur at various discharge levels and can project the resulting outcomes of flow scenarios through time in the form of “habitat time series.” In the parlance of IFIM, microhabitat refers to small spaces or areas up to a few meters in extent. Mesohabitat refers to small areas of the stream and includes depth and velocity of stream flow, substrate characteristics, and vegetation cover, often associated with a geomorphic unit of the stream, such as a pool, run, or riffle. Macrohabitat refers to the longitudinal aspects of the stream important to species, and includes water quality (particularly temperature in many applications), channel morphology, and overall water discharge. Macrohabitat includes a longitudinal portion of stream within which physical or chemical conditions influence the suitability of the entire stream segment for an aquatic organism. The scale of analysis is sometimes kilometers. Total habitat in IFIM models refers to the entire wetted perimeter of the channel along a given length of stream that may extend several kilometers. Geographically explicit predictions can define the expected areas and locations of useful habitat for the species in question because such habitat is a function of discharge. For a complete discussion of IFIM and its relationship to PHABSIM, consult Waddle (2001) or the website version.

IFIM relies on a complex system of hydrological models that operate with computer software (Figure 4-17). Because IFIM requires as input cross-sectional survey data and discharge records and calculates the amount of available habitat for many reaches of the river and for several species (in the Platte River application), its full development required several years to complete. USGS maintains the software and assists in its application by USFWS and other DOI agencies. The software is in the public domain, and it is used by university researchers and private consulting firms for contract work.

During the period when USFWS was determining its instream-flow recommendations for the central and lower Platte River, the agency used IFIM and applied it to the three threatened or endangered avian species along the river. When the agency completed its calculations and made instream-flow recommendations, PHABSIM and IFIM were the state-of-the-art technology for instream-flow purposes and were used for many rivers in the United States, Europe, Australia, and New Zealand (Gillilan and Brown 1997). The software development began in the 1970s; by the 1990s, it had become “the most sophisticated and comprehensive method of quantifying instream flow needs” (Stalnaker et al. 1995). Research by USBR, USFWS, and USGS personnel—including the survey of additional

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-17 Basic operations of Instream Flow Incremental Method (IFIM) used by DOI agencies to define recommended flow magnitudes, frequency, duration, and timing. Source: Waddle 2001.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

cross sections, additional measurements of habitat relationships, and modifications of the programs that make up IFIM—resulted in continuous improvement of the models through the 1990s. Continued improvement of the models makes a “rerun” with original data possible.

Application of IFIM models to the Platte River by DOI agencies produced a series of instream-flow recommendations. A 1990 workshop brought together interested researchers to discuss the problem of establishing instream-flow recommendations, partially stimulated by relicensing requests to the Federal Energy Regulatory Commission for power projects along the Platte River owned by the Nebraska Public Power District and the Central Nebraska Power and Irrigation District (M.M. Zallen, DOI, unpublished material, August 11, 1994). By 1994, DOI agencies had used IFIM to generate their recommendations, and after some revisions the agencies recommended three types of discharges: species flows, annual pulse flows, and peak flows. Later modifications in the recommended target flows resulted from work in 1996, 1999, and 2001. The relevant documents describing the rationale for these flows and for specific recommended values are Bowman (1994), Bowman and Carlson (1994), Altenhofen (J. Altenhofen, Fort Collins, CO, unpublished memo, March 4, 1996), Boyle Engineering Corporation (1999), and Murphy et al. (2001).

At the time of the National Research Council review, the most recent summary of DOI recommended target flows was that of USFWS (2002b). Tables 4-3, 4-4, and 4-5 outline the agency recommendations. Instream-flow targets for general purposes or specific species represent discharge conditions that are intended to result in favorable habitat for the threatened and endangered avian species along the central Platte River and for pallid sturgeon in the lower Platte River (Table 4-3). The recommendations specifically recognize the variability among wet years (the wettest 33% of years on record), dry years (the driest 25% of years on record), and normal years (all others) that result from short-term weather extremes. The general recommendations seek to ensure that water flows in the channel in sufficient quantities to support the species in question but not enough to harm

TABLE 4-3 Species Instream-Flow Recommendations of USFWS for Central Platte River, Nebraska

Period

Wet Year Discharge, cfs

Normal Year Discharge, cfs

Dry Year Discharge, cfs

Jan. 1-Jan. 31

1,000

1,000

600

Feb. 1-Mar. 22

1,800

1,800

1,200

Mar. 23-May 10

2,400

2,400

1,700

Oct. 1-Nov. 15

2,400

1,800

1,300

Nov. 16-Dec. 31

1,000

1,000

600

 

Source: USFWS 2002b.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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TABLE 4-4 Annual Pulse-Flow Recommendations of USFWS for Central Platte River, Nebraska

Recurrence Interval

Recommended Flow, cfs

Notes

3 of every 4 years (75%)

3,100-3,600 (Feb.- Mar.)

3,000 (May-June)

3,400 (May-June)

30-day duration for Feb.-Mar.

7- to 30-day duration for May-June

 

 

10-year running mean of 30-consecutive-day exceedance

Every year (100%)

2,000-2,500 (Feb.-Mar.)

30-day duration for Feb.-Mar.

 

Source: USFWS 2002b.

other listed species. The values are derived from the observation that many habitat values are at their maximum when flows are 2,000-2,500 cfs and that below that value suitable habitat areas diminish in size and become restricted in their distribution. The final values in Table 4-4 represent a compromise between habitat needs and a management objective to store as much water as possible. For specific discussions of the needs of each species, see Chapters 5, 6, and 7.

Pulse flows (Table 4-4) maintain the general functionality of the river by supporting the movement of sediment, the woodland and open-channel balance, and the shaping of the channel by physical processes. The movement of sediment through dryland rivers in pulses is a common observation (Graf 1988), so in returning the Platte River to a condition that is more similar to predam conditions, pulse flows must occur with their natural timing during spring or early summer. Pulse flows must reflect natural duration to be effective and provide some variation in flow conditions—a characteristic that is a departure from artificially controlled flows that remain similar from month to month with little change (Richter et al. 1996, 1997).

TABLE 4-5 Peak-Flow Recommendations of USFWS for Central Platte River, Nebraska

Recurrence Interval

Recommended Flow, cfs

Notes

1 of every 5 years (20%)

16,000 Feb.-June

5-day duration

At least 50% of flow between May 20 and June 20

 

 

May-June preferred

2 of every 5 years (40%)

12,000 Feb.-June

Feb.-June for channel maintenance

10-year running average of 5-consecutive-day exceedance

8,300-10,800 Feb.-June

5-day duration

 

Source: USFWS 2002b.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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All natural dryland rivers are subject to occasional large floods that accomplish considerable geomorphic work by maintaining or changing the channel configuration (Rhoads 1994; Schumm 1977). From a biological perspective, the forms and processes maintained by these infrequent peak flows are the substrate for suitable habitat. USFWS recommended that peak flows be large-discharge events lasting about 5 days (Table 4-5). Their exact timing by year is less important than their occurrence about once during each 5-year period. From a management standpoint, they therefore might be created through controlled releases only during relatively wet periods. The recommended once-every-5-year pulse flows represent flood discharges that are of a general magnitude that is similar to the pre-1909 annual flood flows (the period before construction of large dams upstream). The magnitude of the 10-year running average for recommended pulse flows is about 50-60% of the magnitude of pre-1909 annual flood flows. Therefore, in magnitude they are similar to predam flows, but in the restored river these high flows would be less frequent than the predam flows (compare Tables 4-3 and 4-5).

The literature in geomorphology, hydrology, and ecology supports the basic importance of maintaining species, pulse, and peak flows for river restoration and species protection as applied by USFWS and USBR. In geomorphology, literature defining the importance of general flow maintenance, pulse flows, and peak flows for channel formation and maintenance includes Schumm (1977), Graf (1988), Richards (1982), Leopold (1994, 1997), Brookes and Shields (1996), Knighton (1998), and Petts and Calow (1996a,b). The supporting hydrological literature includes Chow (1964), Shen (1971), Simons and Senturk (1992), and Chang (1998). The supporting ecological literature includes Petts and Calow (1996c), Clark and Harvey (2002), Franklin (1993), Millington and Pye (1994), and NRC (2002a).

The recommended pulse flows are smaller than the floods observed before the 1941 closure of Kingsley Dam, when river processes and flood magnitudes were more natural than they are now. Historical aerial photography, including the paired examples in this report, show that channel shrinkage has occurred since 1941, so small pulse flows are appropriate to the shrunken channel system. The pulse flows are likely to be contained within the banks of the existing river and are therefore unlikely to threaten property damage.

The proposed instream flows that resulted from the DOI agencies’ analysis and that are summarized in Tables 4-3, 4-4, and 4-5 appear to the committee to be in the correct magnitude and timing to achieve the desired results of using river processes to foster habitat for the threatened and endangered species. The flows represent reasonable calculated estimates of the magnitude needed to readjust the channel geomorphology, channel deposits, and channel-side landforms to suit the needs of the species better

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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than the flow regime that results from no management for habitat benefit. The timing of pulse flows reasonably mimics the timing of such events before the installation of large numbers of dams upstream, particularly before the closure of Kingsley Dam. Annual high flows during May are likely to restrict the success of seedlings and thus preserve open spaces needed by the species. Occasional pulse flows will mimic flood events that once partly controlled the river’s geomorphology and vegetation. Specific management questions—such as whether the periods between peak flows are appropriate for maintaining useful habitat system and whether the proposed system of flows is sustainable over a period of many years—cannot be resolved at this time. Monitoring of the system response to the instream flows will be required to inform managers about the efficacy of their management strategies and will suggest possible adjustments—a working example of adaptive management.

Habitat Suitability

Habitat is the physical space within which an organism lives and the biotic and abiotic resources in that space. Habitat is a species-specific or population-specific concept. It is the sum of the resources that are needed by an organism, including those used for forage, shelter, reproduction, and dispersal. The quality of habitat for any given species varies from location to location, from season to season, and from year to year. Organisms are adapted to the features of the environment that enhance their survival. Analysts determine the degree of habitat suitability for a species by assessing the presence and abundance of organisms in relation to one or more physical characteristics of the environment. The suitability of any habitat can therefore be determined by relating species and abundance to habitat features on the assumption that greater density reflects greater habitat suitability. Habitat suitability guidelines have a history in USFWS more than 25 years long and have been formalized in habitat evaluation procedures and their applications in habitat suitability index modeling (USFWS 1980, 1981a,b). Habitat suitability relationships are based on expert opinion and data-collection campaigns that related species to particular environmental conditions. Frequently, common characteristics—such as river depth, velocity, and bedload for fish or vegetation height, density, and composition for birds—are key factors in the process. Relative abundance connected to those environmental variables provides a guide to identifying habitat suitability. PHABSIM is especially beneficial in the process of sorting out habitat suitability because it uses the same ecosystem elements as do the habitat characteristics.

Over the last 2 decades, USFWS developed habitat suitability guidelines for the four listed species in the central Platte River. The guidelines define the physical characteristics of habitats deemed useful to each spe-

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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cies for stopover, resting, breeding, or feeding. The guidelines aid in connecting knowledge about species needs with management strategies designed to enhance or create habitat to support them. USFWS based habitat suitability guidelines on the best available information, even in the cases of the whooping crane and pallid sturgeon, for which available data were particularly sparse. USFWS’s specifications for suitable habitat relied on data collected by DOI personnel, research by them and other professionals, and wide-ranging expert opinion. Chapters 5, 6, and 7 provide complete reviews of the background literature on each of the species. In reaching its conclusions, USFWS relied on 54 pieces of published literature (including many standard reference books and articles in peer-reviewed journals) that form a convincing basis of habitat suitability findings. The agency incorporated several measurable physical-habitat elements—including depth of water, vegetation density, size of sandbars, and river-level variability (water-level fluctuation)—as physical measurable dimensions that are relevant to the listed species. The resulting habitat suitability guidelines offer an estimate of the degree to which particular physical environments can serve as viable habitat for a species. The guidelines can be broadly defined or sensitive, depending on how much is known about the selected species and the degree to which the physical environment can be codified or quantified.

Habitat suitability guidelines for the listed species on the central and lower Platte River emerge from processes outlined by USFWS (unpublished material, June 16, 2000). USFWS defined specific optimal conditions for the whooping crane, piping plover, interior least tern, and pallid sturgeon beginning with the hydrological behavior of the river as related to habitats for the species. For example, in the case of cranes, USFWS identified common characteristics of known roosting sites, such as a wide channel, lack of forest vegetation, sandy substrate, unobstructed long views, and shallow water nearby. For the piping plover and interior least tern, USFWS deduced common characteristics among river-channel habitat, nesting sites, and feeding locations for each species. For the pallid sturgeon, USFWS identified favorable river conditions, including the presence of sandy bottoms, islands or bars, and sediment-rich waters. USFWS extended the earlier study by exploring restoration measures that could re-establish suitable habitat conditions where they had been lost.

Additional Scientific Data Issues for Decision Makers

In addition to the issues of interconnections among ecosystem components, instream flows, and habitat suitability, several issues remain to be addressed by DOI researchers: how species other than the listed ones that use the Platte River corridor are affected by management, alternative

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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approaches to measuring and managing the effects of water-control infrastructure on river flows and other ecosystem components, the physical implications of present restoration efforts, the other ecological characteristics of the system that need to be re-established, and evaluation of water-quality and climate-change issues.

THE PLATTE RIVER AS AN ECOSYSTEM

The Platte River ecosystem is one of the most diverse in the Great Plains, with an especially rich assemblage of vertebrates and higher plants. For example, 58 species of fish live in the central Platte River (Chadwick et al. 1997), and more than 300 species of vascular plants grow on the Platte’s floodplain (Currier 1982). About 50 species of birds nest in the Platte’s floodplain woodlands, nearly half of which are neotropical migrants, a group of birds in decline in other parts of their ranges (Robinson et al. 1995). Perhaps as many as several hundred bird species, including large numbers of waterbirds, use the Platte’s channel and woodland communities twice each year during transcontinental migration (Chapter 2). This bountiful biodiversity is maintained by substantial habitat heterogeneity on the Platte’s floodplain, adjacent wet meadows, and nearby agricultural fields. Heterogeneity in the river system is now controlled largely by the hydrological regime.

The elements of USFWS’s target flows are based on perceived needs of the listed species. For example, a pulse flow is recommended to scour vegetation to provide more suitable sandbar habitat for terns, plovers, and cranes; higher spring and fall flows are proposed to favor whooping crane use during migration; and higher summer flow minimums are proposed to reduce the mortality of forage fish used by terns. However, developing a single set of target flows for one river to favor four very different species (one large migratory bird, two small nesting birds, and one large fish) that have divergent threats, different ecological optima, and different space-related and time-related uses of habitat comes with costs to other species (Clark and Harvey 2002). In addition, specific flow prescriptions that benefit one listed species may actually cause harm or preclude benefits to other listed species. For example, using more water from reservoir storage in fall for whooping cranes may reduce the opportunity to provide adequate spring pulse flows to build higher sandbars for tern and plover nesting.

Criticism of species-focused approaches to develop flow targets for rivers has stimulated the development of alternatives. Most of them emphasize a holistic or ecosystem-level approach that involves returning rivers closer to their predevelopment hydrograph in an effort to regain natural stream-flow variability. Shifting the current hydrograph toward the “natural flow regime” should simultaneously benefit large numbers of native

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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riverine species adapted to preregulation conditions, including those currently protected. This approach should have a lower probability of unintended losses or shifts in biodiversity than reshaping the hydrograph on the basis of the expected response of a small number of rare species.

A related approach to systemic river restoration is that of the “normative river” (Stanford et al. 1996). Normative habitat conditions are those established from what is possible in a natural-cultural context, as opposed to striving for pristine conditions. The primary goal is to regain as much as possible of the former structure of the hydrograph (peaks, pulses, base flows, and timing) given system constraints (storage capacity, water rights, and property damage). The difference between a normative-river approach and that currently proposed by USFWS for the Platte River is that the latter may produce a disarticulated target hydrograph that differs in fundamental ways from predevelopment river flows. USFWS recommended peak-flow target for the Platte, for example, is scheduled earlier than when most predevelopment peaks occurred, possibly to prevent flooding of interior least tern and piping plover nests. The primary advantage of the development of a normative hydrograph is that the ecosystem-based approach emphasizes the rebuilding of key physical processes and high biodiversity of the preregulated river.

The Yakima River Basin Enhancement Project is working toward the establishment of normative flows for salmon (NRC 2002b); this project is a good example of a basinwide riparian restoration project that couples basic research with clear management objectives and stakeholder participation, and it may serve as an alternative approach to that recommended by USFWS for the Platte River. Another river-restoration project after which the Platte could be modeled is that of the Colorado River (Patten et al. 2001). The restoration of the Florida Everglades, which involves reversing the undesired effects of low dams and water-control structures, is based on the acknowledged first step, which DOI agencies label “Get the water right.” A similar overriding objective as a first step also fits well for the Platte River.

In addition to the different outcomes of applying the PHABSIM-IFIM and normative models, the two approaches are underlain by fundamentally different philosophies. IFIM deals with the issue of connecting river flows to physical characteristics of the river one species at a time. It defines the flows needed to create and maintain habitat for one species, then begins again to define the flows needed by a second species, and so on. The problem with that approach is that it obscures the complexity of the real world and instead treats the river as a machine that can be analyzed and “tooled” for the benefit of each species in turn. The normative-flow regime, in contrast, has as its philosophical basis the view of the river as an integrated system. The approach is to control water flows and pulses in a manner that is as close to the “natural” (in the case of the Platte River, the

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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predam or pre-1909) discharges as possible. The normative approach recognizes that the existing river, its aquatic habitats, and its riparian systems are different partly as a result of river engineering (such as bridges) and partly as a result of upstream watershed changes. The idea underlying the normative approach is that if the more “normal” flows are restored, more “normative” habitats will logically follow, and these habitats will have a complexity sufficient to sustain a wide variety of species, including species that are endangered, threatened, and not listed.

The committee recognizes that the DOI agencies have used IFIM because at the time of their decisions it constituted the best available science—a circumstance that lends credibility to their management decisions. To maintain that credibility, however, the DOI agencies must shift their approach to one based on the normative flow regime because it now (2004) constitutes the best available science. In a recent, authoritative review of the relationships among stream flow, sediment, and habitat from the physical science perspective, Pitlick and Wilcock (2001) conclude that “restoration efforts that focus on site-specific issues or single-species enhancement are likely to fall short of their objectives.” Similar sentiments come from the biological community (Poff et al. 1997). Poff et al. (2003) and Richter et al. (2003) provided the most recent statements on the normative-flow approach that can be employed on the Platte River. The normative-flow approach has also seen successful applications in river-restoration efforts in Australia and South Africa (Postel and Richter 2003). One important aspect of restoration of the Platte River concerns the woodland cover in and along the river. The Platte River presents a management conundrum. Riparian woodland established in the central Platte River between the 1930s and 1960s is now in its most productive and diverse stage and supports the majority of species on the floodplain. The clearing of large tracts of this woodland is recommended by USFWS to recover the target bird species. In contrast with other western U.S. rivers where river dewatering has stimulated the expansion of invasive trees of low wildlife value, the Platte’s woodlands are dominated by native species, primarily cottonwood and willow, with high wildlife value. Because riparian areas are positioned at the convergence of terrestrial and aquatic ecosystems, they are hotspots of biodiversity and exhibit high rates of biological productivity in marked contrast with the larger landscape (NRC 2002a).

USFWS has chosen a conservation strategy that includes the removal of riparian woodland in the Platte River to produce more open-channel habitat for the three listed bird species (Figure 4-18). The clearing of wooded islands followed by periodic disking and mowing to keep vegetation short was begun in about 1980 by conservation organizations that used privately raised funds (Lillian Rowe Sanctuary–Audubon Society) and trust-fund earnings (Platte River Whooping Crane Trust). In addition, many wooded

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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FIGURE 4-18 Cleared area along central Platte River. Removal of woodland cover designed to improve long sight lines and open areas to benefit whooping cranes. Source: Photograph by W.L. Graf, May 2003.

islands were removed by being bulldozed to riverbed level. The practice has been effective in attracting larger flocks of roosting sandhill cranes (Faanes and LeValley 1993). Clearing was expanded more recently through expenditure of large amounts of public funds primarily from the Partners in Wildlife Program (a landowner-federal cost-sharing program) and the requirement that power companies clear woodlands to acquire new operating licenses. Exact measures are not available for the extent of woodland clearing in the restoration of the Platte River under present plans. As much as one-third of the 45-mile long intensive management segment may be cleared, with additional areas cleared by Partners in Wildlife. The context of this clearing is that it is focused in a particular segment of the 310-mile river. Clearing smaller parcels for Partners in Wildlife projects and private duck blinds may amount to 25% of the woodland in the less intensively managed river sections.

Clearing as a restoration strategy has both beneficial and adverse outcomes. The adverse consequences of clearing include loss of wooded nesting and migratory habitat for many species of songbirds; proliferation of invasive purple loosestrife due to soil disturbance and chopping of mature plants; possible oversupplying of sediment to downstream reaches, a cause

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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of woodland expansion; loss of cottonwood and willow ingrowth needed to replace future senescent stands (cottonwood and willow are pioneer species that do not reproduce in established woodlands); and reduction in patch size of remaining woodlands. The expected beneficial consequences of clearing wooded areas include improved habitat for the three federally listed bird species of the central Platte as well as many other bird species that require more open habitat (Chapter 2, Appendix B). Other less defined benefits include re-establishing lost or reduced ecological processes that may be important to the proper function of a more natural river system.

Restoration sometimes focuses on benefits to the threatened and endangered species, but unintended detrimental consequences for other species ought to be minimized. In the case of the central Platte River, clearing of woodland to improve habitat for whooping cranes, for example, entails removal of forest environments for some songbirds. The “cost” to songbirds versus the “benefit” to cranes has not been carefully studied or determined. The effects of woodland clearing on other bird species are discussed in Chapter 2.

No quantitative assessments of the response of the listed bird species to clearing over the last 2 decades have been published, but some qualitative patterns are apparent. Sandhill cranes concentrate in wide, unobstructed channels, including areas that have been cleared. For example, in 1999, 66% of the night roosts for about 300,000 sandhill cranes were on reaches of river where forests had been removed (Platte River Whooping Crane Maintenance Trust 1999). The proportion of the whooping crane population that stops on the Platte River during its spring and fall migrations has increased since 1976 (Chapter 5); when cranes stop on the Platte, they often settle near or in cleared areas. In contrast, tern and plover populations have declined steadily in the central Platte since the late 1980s despite increasing cleared area. The reported downstream shifting of sandhill crane populations from the upper to the central Platte in the last 25 years (Faanes and LeValley 1993) has been attributed to upstream channel narrowing. Whooping cranes have also shifted their use patterns to the east (Stehn 2003). The channel area in the upper Platte has increased steadily since the 1950s (although not necessarily in the areas that the cranes have used or to the degree that roosting cranes will find useful), so there may be other factors in the downstream shifting. Other factors include the clearing projects that attract cranes, and the proximity to abundant supplies of waste corn located near suitable roost sites. The issue of shifting use patterns needs more study and analysis.

Few data have been collected in vegetation removal projects that illuminate the effectiveness of clearing. Thus, exactly what is lost and gained through woodland removal is often poorly known. Studies were initiated in the late 1990s at Cottonwood Ranch and Jeffrey Island to monitor the effects of clearing on vegetation, sediment supply, and channel structure. Several years of preclearing measurements, including an inventory of plants

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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and birds, were made before the first clearing began in winter of 2003. It may be 5-10 years before results will be available from these studies to evaluate the practice of clearing. General studies are needed to develop broad standards to define the expected benefits of clearing to open-channel species; to assess the relative success of past clearing; to propose and test alternative woodland-removal methods; and to develop effective approaches to habitat management that best support ecological restoration to the extent possible. Historical data indicate a range of tree densities along the central and lower Platte River before settlement (see cover painting of this book). Therefore, when viewed as a whole, the river in its entirety should reflect a variety of tree densities.

Approaches to restoration in the central Platte have been under way for over 2 decades. Thus far, crane populations have increased during this period, and there is strong evidence that they followed cleared areas for roosting purposes (Faanes and LeValley 1993). Other factors, including availability of upland food sources, may also influence this geographic distribution of birds but to a lesser extent than does roost site availability (Su 2003; Iverson et al. 1987). Management has not stimulated increases in piping plover or interior least tern populations, and their numbers on the Platte have continued to decline. Restoration efforts have also benefited other waterbird species but are likely to have affected woodland species adversely. Restoration of the central and lower Platte River in the future can provide a context for a biologically diverse ecosystem that includes a variety of gradients from forest to open areas. In an adaptive management approach, the maintenance of some woodland along with the open areas will permit an assessment of the effects of restoration on all the species in the ecosystem. A monitoring system is essential to the success of such an ecosystem restoration.

The management of the central and lower Platte River through a partially restored flow regime (a normalized flow pattern) and reduced forest cover in some locations may cause a reduction in some native vegetation species (such as red cedar and hackberry) and some nonnative species (such as honeysuckle and buckthorn). Management of the ecosystem to benefit particular fauna inevitably will affect some species adversely, but the objectives of adaptive management, with its constant monitoring and redefinition of strategies, can minimize the unwanted effects. This approach will allow exploration of the responses of a variety of species to the managed changes and will provide an opportunity to learn more about the role of control factors other than the river, such as fire, predators, and human activities.

SUMMARY AND CONCLUSIONS

This section summarizes the committee’s observations and conclusions about DOI’s approach to understanding the physical processes and forms

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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that underlie the habitats of threatened and endangered species in the Platte River Basin. It begins with a brief summary of the origins and major points in DOI’s approach to river-flow management and habitat connections and then presents the committee’s observations and suggestions. The section concludes with a list of specific recommendations.

USFWS has developed instream-flow recommendations through literature reviews, field observations, data collection and analysis, numerical modeling, workshops, and other approaches. Those processes and methods are scientifically valid, and the techniques applied in the Platte River continue to be used for many other rivers. DOI-recommended flow values appear reasonable, but their effects on this river system require further analysis based on empirical data collection and field observations. USFWS has already expended a great deal of effort to develop an effective flow-management plan, and more investigations are planned. According to USFWS (2002b), “these flow recommendations are intended to achieve the flow-dependent goal of rehabilitating and maintaining the structure and function, patterns and processes, and habitat of the central Platte River Valley ecosystem.” Four types of flow recommendations were made: for species flows, annual pulse flows, peak flows, and program target flows. The values of the species flows for dry, normal, and wet years were based on a consultation process initiated in the 1980s and concluded with the discussions in the March 8-10, 1994, workshop, summarized by David Bowman (1994). The values of the annual pulse flows and peak flows for dry, normal, and wet years were presented by Bowman and Carlson (1994) and based on a workshop held on May 16-20, 1994. Flow values for the Platte River were based on expert opinions summarized in the two reports. According to USFWS (2002b), target flows are the discharges that the program activity seeks to establish through the water-control infrastructure to alter magnitude and timing of flows.

The central Platte River contains both meandering and braided reaches that represent complex hydrological and geomorphic conditions. The current state of knowledge is insufficient to predict its precise morphologic change due to flow volumes with confidence, and a great deal of field observation is needed to support the analysis. The analysis should experiment with series of flows designed to meet the variety of requirements related to vegetation growth, channel maintenance, sediment mobility, and ecosystem stability. It is essential that the field data be collected and analyzed to evaluate the actual effects of USFWS-recommended flows on the central Platte River, should they be implemented.

Monitoring river behavior requires careful design of field data collection. Specific data-collection characteristics—location, timing, goals, and level of detail—should be planned well before the occurrence of the targeted flow events. Field data can be expensive to collect, but timing is

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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important because large flows are highly uncommon. If a research program does not collect data during and after a large flow, the next opportunity may not occur for many years. Perhaps USFWS could use its workshop technique to discuss various field needs of different disciplines. DOI flow recommendations would be most helpful if they were evaluated for suitability for listed and other regionally important species.

Regarding DOI interpretations of the interrelationships among flow, sediment, morphology, and vegetation, the committee has two sets of observations, one concerning the channel and history work by Murphy and Randle (2003) and the other concerning the sediment and vegetation work being undertaken by Murphy et al. (2001). First, undoubtedly, there are strong relationships among sediment, flow, vegetation, and channel morphology. Flow is also directly related to climate, water needs, reservoir storage, and diversion. Since the construction of the first large dam (Pathfinder in 1910), various water-resources developments have altered flow distribution and water consumption substantially, especially in the upper Platte River system. Active discussions between the Environmental Impact Statement team and the Parson team occurred in our August committee meeting in Grand Island, Nebraska, about the relative importance of climate and the water-resources developments for the change of river characteristics (for examples see Parsons 2003; Lewis 2003; Woodward 2003; Yang 2003; Murphy and Randle 2003).

Water-resources developments have diverted and returned flows into the North Platte River, South Platte River, and upper Platte River. Murphy and Randle (2003) have estimated consumptive uses of water, including sewage, and evaporation of reservoir water. Because the return flows from diversions (except Kearney Canal return) occur upstream near Overton, Nebraska, the relative flow effect of water-resources development is considerably greater on the upper Platte River than on the central Platte River. Regardless of climate change, water-resources development will continue to affect Platte River flows as long as there is a net irrigation water consumption and reservoir evaporation. The human controls on flows in the river are the most important controls on a daily, monthly, or annual basis, but the longer-term effects of climate change are a background control worthy of further investigation.

The mathematical modeling by Murphy et al. (2001) has some shortcomings that will challenge DOI investigators. Some of the required data are unavailable, and some of the modeling techniques are still in the development stage. The success of a numerical model depends on knowledge of flow roughness in the flow-momentum equation and of sediment transport rate in the sediment-continuity equation. Methods in field data collection to improve that knowledge have yet to be fully developed. The relationships of

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
×

lateral variations of flow properties among the different subsections in a braided river are also difficult to determine.

There are four dimensions in any river system: the longitudinal direction, which is the main flow direction; the lateral direction, across the channel into the floodplain; the vertical direction, including surface-groundwater exchanges; and time. In a one-dimensional model, we investigate only the change in flow properties in the longitudinal direction with time and space by assuming no change in the other dimensions. Thus, in a strictly one-dimensional model, the lateral cross section is uniform because we assume no change of flow in the lateral direction. In a braided channel, a river cross section (perpendicular to the main flow direction) may include two flow channels with an island in between. Thus, Murphy et al. (2001) use a pseudo-one-dimensional model to include the possibility of having a channel cross section with nonuniform shapes. That approach is generally accepted, and many models use different schemes to represent sediment and lateral flow distribution among the various lateral cross sections. Several unproven assumptions have been used for the lateral distributions of flow and sediment in the current pseudo-one-dimensional model. The vegetation resistance should be determined from the field data instead of from other references. Some of the longitudinal intervals between two cross-sectional stations are too long to yield any reliable hydrogeomorphic relationships. If properly calibrated and validated, this model can give qualitative impressions of sediment and flow analyses, including the evaluation of the effect of vegetation removal and management.

Only a few two-dimensional mathematical models that can include sediment movements are under development. Two-dimensional models have limited application because two-dimensional flow data are often unavailable to calibrate them.

The committee recognizes six approaches for potential improvement of DOI investigations into ecosystem dynamics on the central and lower Platte River:

  • Field data collection and methods for the monitoring of the effects of various flow recommendations and mechanical removal of vegetation must be carefully designed long before the occurrence of the targeted flow events or vegetation manipulations. Some kinds of data are also essential for calibrating and verifying the mathematical model.

  • A risk-based hydrological model should be explored with various penalty functions (water not diverted to users as previously has been the case) at the water-demand points for optimization analysis of the flow-management plan of the river system. The effects of mechanical removal of vegetation should be included in the flow-management plan.

Suggested Citation:"4 Scientific Data for the Platte River Ecosystem." National Research Council. 2005. Endangered and Threatened Species of the Platte River. Washington, DC: The National Academies Press. doi: 10.17226/10978.
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  • For more detailed analysis, variations in flow velocity and flow depth are more important than flow discharge for evaluating ecological requirements because it is the depths and velocities that create habitats.

  • Climate and water-resources developments can have strong influences on river flow distributions. Water-resources development affects river flows substantially in the upper Platte River, and its effects extend to the central Platte River. The relative importance of climate influences and water-resources development on channel characteristics should be analyzed and should encompass a record of several decades.

  • Restoration of the central Platte River should include water processes and forms, control of invasive species, and some grazing and fire if research shows these phenomena to be important aspects of the pre-European river.

  • More emphasis should be placed on the management of the Platte River as an ecosystem, rather than keeping the focus exclusively on listed species.

In summary, the committee’s review of DOI’s efforts to explain and model the connections among ecosystem components of water, sediment, morphology, and vegetation leads us to conclude that these efforts are underlain by valid science. Likewise, DOI’s instream-flow requirements are grounded in scientific understanding of the system and in the technology of model construction that was state-of-the-art when the decisions and recommendations appeared. Science and engineering are making progress, however, and new technology is becoming available. New advances are needed because of the braided, complex nature of the Platte River, a configuration that is unlike that of the streams where others often apply the models. Current DOI model developments, including the emerging SEDVEG model, are likely to be helpful and useful in both understanding and managing the Platte River. DOI’s determination of suitable habitat rests on the best available science. The committee also recognizes, however, that there has been no substantial testing of the predictions of DOI’s modeling work,1 and we urge that calibration of the models be improved and that monitoring of the effects of recommended flows and vegetation management be built into a continuing program of adaptive management. In such a system, monitoring can indicate whether recommendations and determinations are valid and can suggest further adjustments to the recommendations and determinations on the basis of observations.

1  

The committee did not consider USGS’s in-progress evaluation of the models and data used by USFWS to set flow recommendations for whooping cranes.

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The tension between wildlife protection under the Endangered Species Act and water management in the Platte River Basin has existed for more than 25 years. The Platte River provides important habitat for migratory and breeding birds, including three endangered or threatened species: the whooping crane, the northern Great Plains population of the piping plover, and the interior least tern. The leading factors attributed to the decline of the cranes are historical overhunting and widespread habitat destruction and, for the plovers and terns, human interference during nesting and the loss of riverine nesting sites in open sandy areas that have been replaced with woodlands, sand and gravel mines, housing, and roadways. Extensive damming has disrupted passage of the endangered pallid sturgeon and resulted in less suitable habitat conditions such as cooler stream flows, less turbid waters, and inconsistent flow regimes. Commercial harvesting, now illegal, also contributed to the decline of the sturgeon.

Endangered and Threatened Species of the Platte River addresses the habitat requirements for these federally protected species. The book further examines the scientific aspects of the U.S. Fish and Wildlife Service’s instream-flow recommendations and habitat suitability guidelines and assesses the science concerning the connections among the physical systems of the river as they relate to species’ habitats.

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