Missouri River and Floodplain Ecology
I observe a great alteration in the Current course and appearance of this pt. of the Missouri. in places where there was Sand bars in the fall 1804 at this time the main current passes, and where the current then passed it is now a Sand bar. Sand bars which were then naked are now covered with willow several feet high. the enterance of some of the Rivers & creeks changed owing to the mud thrown into them, and a layor of mud over some of the bottoms of 8 inches thick.
Captain William Clark, August 20, 1806
The Missouri River ecosystem experienced a marked ecological transformation during the twentieth century. At the beginning of the century, the Missouri River was notorious for large floods, for a sinuous and meandering river channel that moved freely across its floodplain, and for massive sediment transport. But by the end of the twentieth century, the Missouri River bore little resemblance to the previously wild, free-flowing river. This chapter describes these environmental changes in the Missouri River ecosystem, contrasting the ecological state of the river and its floodplain before and after the construction of the series of large mainstem dams and reservoirs. The physical and ecological units of today’s Missouri River ecosystem are then described, followed with a review of the scientific research that forms the basis for our understanding of the ecosystem’s dynamics and the consequences of human actions. The chapter concludes with an overview of the system’s ecological issues and a commentary from the committee.
Seven huge dams were constructed along the Missouri River during the twentieth century, six of them pursuant to the 1944 Pick–Sloan legislation. The river’s annual flooding regime was nearly eliminated in those areas under the dam’s controlling influences. In another key twentieth century environmental change, the river was confined to a single, uniform channel. It was fixed in place by dikes and revetments downstream from just below
Gavins Point Dam, the most downstream dam, to the river’s mouth at St. Louis.
Ecological changes that accompanied changes in hydrology proceeded more slowly but were of a similar magnitude. Large floodplain areas along the upper Missouri were inundated by the reservoirs. Large areas of native vegetation communities in downstream floodplains were converted into farmland. Many native fish and avian species experienced substantial reductions, while nonnative species—especially fishes—thrived in some areas.
Scientists and citizens today understand more fully the consequences of similar changes that have occurred on many of the world’s large rivers and the possibilities for reversing them. In the second half of the twentieth century, the field of large river ecology emerged to provide a scientific basis for river restorations, strategies, and initiatives. This research and the scientific understanding of river ecosystems builds upon a longer history of research on hydrology, geomorphologic processes (the shaping of river channels by water and sediment), vegetation dynamics, and river mechanics. The importance of extreme climatic events in the ecological structure and functioning of large river-floodplain ecosystems features prominently in contemporary river science theories (Bioscience vol. 45, # 3, 1995). Recent studies of large river systems emphasize the ecological values of hydrologic connections between a river’s main channel, backwaters, and floodplain (Gore and Shields, 1995; USGS, 1999; Ward and Stanford, 1995). Aquatic ecosystem restoration practices build upon and complement these theories. For example, current science-based river management paradigms and practices seek to take advantage of the tendency of these large river systems, through natural processes like floods and the transport of sediment, to make and sustain these connections (Bayley, 1995; USGS, 1999). Restoring some ecological functions of a large river system also provides the benefits of maintaining species biodiversity in the river and on its floodplain, as well as restoring habitat for threatened and endangered species. This chapter introduces and explains concepts that underpin the theories and practices of contemporary large river science and restoration. This knowledge helps explain the ecological consequences of human-induced changes in the Missouri River ecosystem and provides the scientific basis for considering public policy decisions to improve ecological conditions.
THE PRE-REGULATION MISSOURI RIVER
Physical Processes: Hydrology and Geomorphology
During the period of westward expansion in the United States, the Missouri River was a large floodplain river. It periodically overflowed its
banks and water spread across its floodplain, hydrologically connecting the channel, to its floodplain and backwaters. The Missouri River created new channels as its main channel moved laterally across its floodplain. These types of geomorphic changes in the channel were a notable feature of the pre-regulation Missouri, especially below present-day Yankton, South Dakota downstream to the confluence with the Platte River. In this stretch of the river, the distance between the Missouri River bluffs ranges from five to eighteen miles (Schneiders, 1999).
Variations in the river channel’s location, form, and volume of sediment transported were driven by changes in river flows. In turn, the river’s hydrology was greatly influenced by weather extremes. The annual pattern of flows on the pre-regulation Missouri included spring and summer rises that generally occurred in April and June, respectively. The April rise was caused by local snowmelt on the plains and by local rainfall; the June rise was caused by melting snowpack in the Rocky Mountains and rainfall at lower elevations. The spring rise tended to be brief, lasting about one to two weeks, and was relatively localized. The summer rise lasted longer and inundated larger portions of the floodplain.
Prior to regulation, the Missouri River was known as the “Big Muddy,” as it carried large amounts of sediment. Erosion tended to be most severe as flood waters were rising, with substantial deposition of sediment occurring as flood waters receded. The Missouri River existed in a dynamic equilibrium with its floodplain, frequently redistributing sediment between its channel and floodplain, as described below.
As Missouri River flows increased in the spring and summer (referred to by hydrologists as the “rising limb” of a hydrograph), the river would erode sediment from its bed and its banks. The river bed would undergo rapid physical changes during this period; it would be degraded (lowered) because of erosion, the channel would migrate laterally, backwaters and the main river channel would be connected by overbank flows, and shoreline and riparian vegetation, including trees, would be scoured and washed into the channel. The rising water also would replenish the groundwater table, an important process for maintaining floodplain vegetation. As flows receded (the hydrograph’s receding limb) and water volume and velocity decreased, the degraded channel would refill with deposited sediment, braided channels and meanders would become isolated from the main channel, and fresh substrates would be deposited for occupation by plants and animals.
Prior to channelization and flow regulation, the lower Missouri River was braided to highly sinuous, a form naturally found in rivers with broad floodplains and heavy sediment loads. Hiram Chittenden described the
river’s pre-regulation sinuosity in his account of the adventures of nineteenth century Missouri River steamboat captain Joseph La Barge:
The river is like a great spiral stairway leading from the ocean to the mountains. A steamboat at Fort Benton is 2565 feet—two and one-times the height of the Eiffel Tower in Paris—above the level of the sea; yet so gentle is the slope nearly all the way that, in placid weather, the water surface resembles that of a lake. This wonderful evening-up of the slope of the river by the extreme sinuosity of its course is a fact not only interesting as a natural phenomenon, but of the utmost importance in the behavior and use of the stream (Chittenden, 1962).
The pre-regulation river was characterized by log jams, snags, whirlpools, chutes, bars, cut-off channels, and secondary channels around bars. The main channel typically had a deep thalweg (the deepest part of the river) that contained the faster-moving flow and a shallower section(s) on one or both sides of the channel (Figure 3.1). The cross-sectional shape of the main channel often exhibited a highly nonuniform velocity distribution (Hesse, 1993). The main river channel’s width was variable, ranging from roughly 1,000 to 10,000 feet wide during normal flow periods to 25,000 to 35,000 feet wide during floods (Schneiders, 1999). Based on surveys completed between 1878 and 1892, the maps in figures 3.2 through 3.4 portray the sand bars, meanders, and extensive floodplain vegetation that characterized the pre-regulation riverine ecosystem at Bismarck, North Dakota, Yankton, South Dakota, and St. Joseph, Missouri, respectively.
River depth was greatest in spring and early summer and shallowest in December and January. However, there were scour holes downstream of
log jams and other obstructions, at bends, or where tributaries entered. Depths in these holes varied from a few inches to more than thirty feet (Schneiders, 1999). Mid-channel and point bars were found along the entire length of the Missouri River. The bars shifted frequently in response to changing flows with the larger bars scoured at higher flows.
The rich biodiversity (see Box 3.1) of the pre-regulation Missouri River ecosystem was sustained through a regime of natural disturbances that included periodic floods and attendant sediment erosion and deposition. These disturbances, in turn, supported a variety of ecological benefits, including commercial and recreational fishing, timber, biomass fuels, wild game, trapping and fur production, clean water, medicines, soil replenishment processes, and natural recharge of groundwater. The ecological pro-
cesses sustained by periodic flooding in river-floodplain ecosystems are encompassed by the contemporary concept known as “the flood pulse.”
The Flood Pulse Concept
The concept of the “flood pulse” summarizes the effects on biota of the connections between the river channel and floodplain (Junk et al., 1989; Bayley, 1995). The flood pulse describes the predictable rising and falling of water in a natural river-floodplain ecosystem as the principal agent controlling the adaptations of most of the biota. Central to the flood pulse concept is the notion that floods, rather than representing a disturbance to the ecosystem, are part of the natural hydrologic regime, and that the prevention of floods actually represents an ecological disturbance (Bayley, 1995). The flood pulse is essential to the health of river-floodplain ecosystems for the following reasons:
Floods add dissolved and particulate organic matter and mineral nutrients to aquatic and terrestrial ecosystems. The river channel and its floodplain both depend on erosion and deposition associated with the channel’s lateral migration. Inundation deposits silts and nutrients that replenish floodplain pools and backwaters. The flooding of terrestrial mineral and organic matter releases nutrients to the water.
Many plants rely upon inundation for rapid growth and reproduction. Species such cottonwood and willow are highly dependent upon periodic floods.
Many animals (invertebrates, fish, birds, mammals) are adapted to the flood cycle and depend upon the high plant and microbial activity associated with it. Floods provide reproductive cues for many fish species in river-floodplain ecosystems. Furthermore, floods make inundated floodplain vegetation available as a food source for fish and invertebrates.
Biodiversity, the shortened term for biological diversity, is commonly used in the lexicon of twenty-first century environmental sciences. The Oxford Dictionary of Biology describes biodiversity as “the existence of a wide variety of species (species diversity) or other taxa of plants, animals, and microorganisms in a natural community or habitat, or of communities within a particular environment (ecological diversity), or of genetic variation within a species (genetic diversity). The maintenance of a high level of biodiversity is important for the stability of ecosystems.” Biodiversity is often associated with the number and kinds of species at a locality. Ecologists describe the number of different species as species richness, while species diversity accounts for both the number of species as well as their relative abundance in a community. It is important to remember that biodiversity includes not only these components, but that it also refers to genetic diversity and may include all manner of organisms ranging from paddlefish to bacteria. Ecosystems that support high species diversity often also demonstrate high biodiversity, such that management for species diversity and species richness in natural ecosystems tends to foster the other, sometimes more obscure, elements of biodiversity. A key change that occurs in regulated river systems with respect to biodiversity is the proliferation of nonnative species. Described by Stanford et al. (1996) as the most pervasive result of habitat alteration in large regulated rivers, this shift usually occurs in communities ranging from fish and invertebrates to riparian and floodplain vegetation. A committee of the National Research Council (2000a) recommended that an indicator that measures native species diversity is an important indicator of human impacts on the environment.
Appreciation of the flood pulse’s ecological significance has grown with recognition of the importance of flooding at various scales and magnitudes (Anderson et al., 1996; Wetzel, 2001). Key aspects of floods are frequency, duration, magnitude, and timing. The combination of these variables controls plant and animal life associations along rivers and influences all aspects of the riverine food web (Poff and Ward, 1989; Richter et al., 1996; Walker et al., 1995).
The timing of overflow is important to native biota adapted to life in rivers and adjoining riparian ecosystems. Periodic high flows and low flows help maintain the health of large river-floodplain ecosystems by acting as “reset” mechanisms that reinitiate early successional vegetation and serve to limit certain faunal associations that can outcompete species normally restricted to life within a channel (Cummins et al., 1984). Fish spawning, insect emergence, and seed dispersal are commonly triggered by
rising waters that, once receding from the floodplain, provide food sources or seed beds for many riverine species.
Because these processes sustained the river’s biological production and diversity, the pre-regulation Missouri River exhibited a rich heterogeneity of habitat. A typical cross-section of the pre-regulation Missouri River contained a deep channel, multiple side channels, oxbow lakes, islands, sand bars and dunes, and backwater habitats interspersed by areas of higher land. These channels and backwater areas provided slower-moving waters critical for the reproduction, shelter, and feeding of fish species. Higher lands contained rich forests, prairie grasses, and thick underbrush that contained a myriad of plant species. Lewis and Clark described this rich pre-regulation biodiversity, noting that the Missouri “nourishes the willow-islands, the scattered cottonwood, elm, sycamore, linden, and ash, and the groves are interspersed with hickory, walnut, coffee-nut, and oak” (Lewis and Clark expedition, Coues, editor; cited in Schneiders, 1999, p. 35).
THE POST-REGULATION MISSOURI RIVER
Physical Processes: Hydrology and Geomorphology
Flow regulation and channelization substantially changed the Missouri River’s historic hydrologic and geomorphic regimes. The primary change was that the extreme high and extreme low flows were lost from the hydrograph downstream of each mainstem dam. This dampening effect below Gavins Point Dam extends downstream to near Nebraska City, Nebraska (Hesse, 1994), where tributary influences partially restore pre-regulation flows to the river. Below Fort Peck Dam, for example, the median high flow was cut in half following the dam’s closure (Shields et al., 2000). Not only have high flows been markedly reduced in many areas, low flows have increased considerably. The result of these changes is an annual hydrograph that exhibits far less variability. Figures 3.5 and 3.6 show hydrologic changes in the post-regulation river at Sioux City, Iowa (just downstream of Gavins Point Dam) and at Hermann, Missouri. These figures illustrate hydrologic changes associated with the construction of the mainstem dams. The figures also show that these changes that occurred are not uniform along the Missouri River and that some downstream sections in the state of Missouri have experienced less hydrologic change with regulation. But the variability that characterized pre-regulation Missouri River hydrology has greatly diminished along most of the river—especially in those reaches directly below the dams—and the spring and summer rises no longer occur in many stretches.
The Missouri River also has experienced large changes to its channel structure and dynamics. The channelized portion of the Missouri River
begins near Nebraska’s Ponca State Park (just upstream of Sioux City, Iowa). From Ponca State Park downstream to the Big Sioux River, the river channel is stabilized. The navigation channel then begins at the mouth of the Big Sioux River at Sioux City. The navigation channel, which extends to St. Louis, ranges in width from 600 feet at the Big Sioux confluence to roughly 1,100 feet at St. Louis. Nearly all channelization activities have been conducted as part of the federal Missouri River Bank Stabilization and Navigation Project (passed as part of the 1945 Rivers and Harbors Act), which is executed by the Corps of Engineers. Channelization has been accomplished through a combination of engineering structures, including hardpoints, a variety of revetments, dikes, and sills (Slizeski et al., 1982; see Figure 3.7).
The cross-sectional shape of the Missouri’s channelized portion (735 miles or about one-third of the river’s length) is approximately trapezoidal. Prior to channelization, the river’s flow had been swift only in its thalweg (a line connecting the deepest points of the river channel), as the river contained sloughs, sandbars, and side channels. But today the river runs swiftly throughout the entire channelized, uniform cross-section. The reduction in width, along with a decrease in flow resistance because of the uniform cross-section and the clearing of snags and sand bars, has caused an increase in flow velocity, which today measures roughly three miles per hour at usual levels of river discharge (Schneiders, 1999).
Regulation of the Missouri River’s flows also changed sediment transport dynamics. Prior to regulation, the amount of sediment transported past Omaha, Nebraska ranged from 228,570,000 metric tons in 1944 to 39,909,297 metric tons in 1931. From 1940-1952 (the period from the closure of Fort Peck Dam until the closure of Gavins Point Dam), the average annual sediment load transported past Omaha was 148,930,000 metric tons. After 1954, the average sediment load was reduced to 29,487,600 metric tons (Slizeski et al., 1982).
Several inter-related processes have caused post-regulation changes in the river’s rate of lateral migration. The channel downstream of the dams has degraded (deepened). Degradation occurs as a result of the water released from the dams and increased currents caused by structures that have been installed to force water into a single channel. These features downstream of dams mimic the natural tendency of flow to mobilize and transport sediment. However, once mobilized and transported downstream, there is no longer an upstream source of sediment to replace sediment removed by these flows. Replacement sediment that would have maintained the dynamic equilibrium of the channel is deposited in upstream reservoirs. But there is an abundance of sediment in floodplain areas lateral to channels below the dams that could be used to meet sediment needs in the channel.
With the exception of Oahe and Big Bend dams, where downstream reservoirs extend upstream nearly to the tailwaters of these dams, channel degradation has occurred below the dams. In the channelized section from Ponca State Park downstream to St. Louis, channel degradation occurs downstream from Sioux City, Iowa to just above the Missouri’s confluence with the sediment-laden Platte River. Farther downstream, especially near the confluence of the Missouri and Mississippi rivers, the channel bed is gradually aggrading. From the confluence upstream to approximately Missouri River mile 12, Missouri River navigation channel depths are frequently impacted by Mississippi River flows, causing a backwater effect up the Missouri River that results in reduced velocities and temporary deposition (Mellema, 1986).
Channel degradation below the Missouri River’s mainstem dams is well documented (Holly and Karim, 1986; Mellema and Wei, 1986; Osterkamp and Hedman, 1982; Sayre and Kennedy, 1978). For example, Figure 3.8 shows that degradation on the order of ten feet or more extends many miles downstream of Garrison Dam. Similarly, below Gavins Point Dam, the river in 1980 had degraded by 8.5 feet.
Degradation of the river channel disconnects the river channel from its floodplain. Channel degradation not only makes it more difficult for the river to overflow its banks, but it also affects the floodplain water table. Most importantly, the lack of flooding removes a source of periodic recharge water for infiltration to the groundwater table. In addition, because the water table (an alluvial aquifer) is hydrologically connected to the river channel itself, there is a consequent lowering of this aquifer in association with the lowering (incision) of the river channel. This lowering of the water table effectively drains water from oxbow lakes and wetlands. Moreover,
in highly-regulated stretches of the river, reduced fluctuations in river stage have resulted in reduced fluctuations in the floodplain water table. These fluctuations are important to maintaining animal and plant species richness in the floodplain, as some species will benefit from a raised water table, while other species will benefit when the water table is lower.
Channelization of the lower Missouri River and subsequent degrada-tion of the river channel also have affected tributary streams. In upstream areas, downward incision is occurring along many of the Missouri’s tributaries. This process occurs because the slope of the tributary channel bed increases in order to meet the (relatively) newly-lowered elevation of the Missouri River channel bed. Many tributaries continue to adjust to the new river bed elevation.
As channel degradation continues to entrench the stream, there are fewer overbank flows than there were prior to degradation, thus reducing interaction between the flow in the channel and floodplain. Rates of channel migration also have decreased. Lateral migration of river channels can occur in areas below dams; however, meandering rates have been markedly reduced downstream of the Missouri mainstem dams because of sharp reductions in peak flows and the armoring of streambanks. Johnson (1992) found that channel erosion and deposition rates (both indicators of river meandering rate) are only 25 percent and 1 percent of pre-dam values, respectively, downstream of Garrison Dam. Similarly, Shields et al. (2000) found that the mean rate of channel migration just downstream of Fort Peck Dam declined from 20 feet per year to 6 feet per year.
Many processes essential to maintaining ecological integrity have been altered in the post-regulation Missouri River. The spring and summer floods have been eliminated in many stretches of the river (although floods still occur in much of the river’s channelized section, especially in downstream sections in the State of Missouri). The isolation of the Missouri River from its floodplain caused by river regulation structures has in many stretches largely eliminated the flood pulse and its ecological functions and services. In these areas, the absence of overbank flooding removes a source of water for the growth of vegetation, as well as a medium for fishes to move into floodplain areas to spawn and feed.
As a result of these changes, the production and the diversity of the ecosystem have both markedly declined. One of these impacts is a reduced ability for trees to regenerate. On the Missouri River and many of its tributaries, this has especially been the case for the cottonwood, largely as a result of the current low rate of river meandering (Johnson et al., 1976; Johnson, 1992). The habitat through a typical cross-section of the post-
regulation Missouri (in the non-submerged portions) has been greatly simplified (Figure 3.9). Side channels and backwater areas have been greatly reduced, thereby eliminating important habitat for many species of fishes, birds, and game. The water, sediment, and nutrients previously spread across the floodplain by overbank flows and the meandering river are now primarily restricted to the main channel or contained in the system’s reservoirs. These changes, combined with other human activities in floodplain areas, have produced an ecologically impoverished ecosystem.
MISSOURI RIVER ECOSYSTEM PHYSICAL AND ECOLOGICAL UNITS
Scientific investigations today are conducted in an ecosystem that changed greatly during the twentieth century and that today is fragmented into distinct physical and ecological units. The mainstem dams, along with other flood damage reduction and navigation enhancement projects, partition the river into four sub-units that differ greatly in hydrology, sediment balance, and biota (Figure 3.10). The river can be classified into four sub-units:
Free-flowing (upstream of the reservoir system);
Remnant floodplains (between the reservoirs);
Channelized reach (downstream of the dams and reservoirs; the lower one-third of the river);
These categories vary markedly in the degree to which they mimic the pre-regulation Missouri River. The mainstem Missouri River is further
influenced by alterations on its tributaries. Dozens of tributaries enter the Missouri River along its course and most have experienced flow and sediment alterations by dams, water diversions, channel modifications, and land use changes to their watershed (e.g., farming and wetland drainage). The transformation of the Missouri River from a free-flowing to regulated river makes the upper basin tributaries, which are generally less regulated in comparison to lower basin tributaries, important components of a comprehensive, basinwide strategy for ecosystem restoration. Some examples are instructive.
Missouri River Tributaries
The Yellowstone River flows 675 miles through Montana to its confluence with the Missouri River at the North Dakota border (Figure 3.10). The Yellowstone River is the longest free-flowing river remaining in the contiguous United States. There are no significant impoundments on the Yellowstone’s mainstem, but nearly one-third of its drainage area has been dammed and six low-head dams on the main channel divert water for
irrigation (Helfrich et al., 1999). While irrigation withdrawals and tributary dams affect the river’s hydrology and low-head dams restrict upstream movement of some native fishes (Helfrich et al., 1999), the river retains much of the ecological character it exhibited prior to European settlement (Jackson, 1994). At their confluence, the flow of the Yellowstone River is greater than that of the Missouri.
Where the Yellowstone River, with its abundant silt load and naturally varying hydrology, meets the Missouri River near the Montana-North Dakota border, near pre-regulation conditions exist. In fact, the Yellowstone River serves as a refuge for many of the Missouri River’s native, warm-water fish (Ryckman, 2000). For example, there are high levels of paddlefish reproduction in the lower Yellowstone in years with above average streamflow. Additionally, native suckers and chubs—in decline throughout much of the river system—are fairly abundant and reproduce in the confluence area (Ryckman, 2000). Moreover, the Missouri River downstream of the confluence is a healthy riparian zone that includes ample cottonwood and willow generation maintained by floods and sediment contributed by the Yellowstone.
The unregulated Bad River empties into the Missouri River at Fort Pierre in central South Dakota just upstream of Lake Sharpe (Figure 3.10). The Bad River is small and intermittent and therefore provides only limited ecological benefits to the Missouri River mainstem when compared to the flows of the Yellowstone River. Moreover, there is only a short distance between the confluence of the Bad and Missouri rivers and Lake Sharpe, leaving little of the sediment-deprived Missouri River downstream of Oahe Dam to benefit from the Bad River’s input of sediment-laden water.
The proximity of the Bad-Missouri confluence to Lake Sharpe has caused much of the Bad River’s delivery of 3.25 million tons of sediment per year to remain near its mouth, reducing channel capacity and increasing flooding in and near Fort Pierre (Thelen and Noeske, 1996). Flow releases from Oahe Reservoir (a few miles upstream of the Bad-Missouri confluence) intended to transport sediment farther into Lake Sharpe cost $12.5 million annually from foregone power revenues (Thelen and Noeske, 1993). The Bad River’s high sediment transport rate results from a combination of highly erodable soils and failure to use best management practices on cropland and rangeland in the watershed (Stukel and Madsen, 2000). Better farming and ranching practices could lessen, but not stop, the sedimentation problem at the confluence and in Lake Sharpe. Thus, the condition of the Missouri River mainstem in the Pierre-Fort Pierre area depends largely on human activities in this tributary watershed.
The Platte River enters the Missouri River near Plattsmouth, Nebraska (Figure 3.10). While the Platte’s upper tributaries (South and North Platte rivers) are highly regulated and used for irrigation water, the relative lack of storage reservoirs on the Platte River itself allows considerable amounts of sediment to enter the Missouri River at the confluence. Grain sizes of this sediment range from coarse to fine sand. The sizeable increase in the Missouri River’s bedload increases the potential for in-channel bar formation and alluviation on the floodplain during floods. The sediment and flow variability added to the Missouri River by the Platte River offer the potential for improving river ecology; but this potential is limited because of the Missouri River’s channelized and highly regulated state both above and below its confluence with the Platte.
In contrast to the Bad and Yellowstone rivers, the Kansas River is heavily regulated (Figure 3.10). Eighteen reservoirs, with a total flood-control capacity of 7.4 million acre feet, have been constructed on the Kansas River (Perry, 1993). These reservoirs are intended to reduce flood damages and to enhance navigation flows on the Missouri River. Although the Kansas River is large enough to affect conditions in the Missouri River below the Kansas River’s mouth, flow regulation and sediment trapping by its reservoirs reduce the potential of the Kansas River to improve ecological conditions in the Missouri River.
Physical Units of the Regulated Missouri River
The only free-flowing reach of the Missouri River lies in Montana, upstream of the mainstem dams. This reach without storage reservoirs extends from the Missouri River source near Three Forks, Montana, downstream to Canyon Ferry Reservoir, a distance of about 30 miles. However, the much longer reach from Canyon Ferry Dam to Fort Peck Lake is only mildly regulated because of the comparatively small storage capacity of Canyon Ferry Reservoir relative to total river flow and the long distance between Canyon Ferry Dam and the next downstream reservoir (Fort Peck).
Contributions from small mountain streams and springs help retain some of the natural flow and temperature patterns in this reach as well. These moderately regulated reaches have retained their essential pre-regulation characteristics, including overbank flooding, adequate sediment supply
to prevent channel degradation, scattered populations of cottonwood forests similar to those observed by Lewis and Clark, and productive native fisheries (Scott et al., 1997). Floodplain vegetation in these reaches is often impacted less by river regulation than by local land use practices, such as grazing (Auble and Scott, 1998).
Remnant floodplain sub-units occur between reservoirs (Figure 3.10). The length of these reaches varies considerably. In some cases, the headwaters of the mainstem reservoirs extend nearly to the tailwaters of the next upstream dam; there are few remnant floodplains from Lake Oahe downstream to Fort Randall Dam. In other cases, reservoirs are separated by large stretches of river (e.g., section 3, from Fort Peck Dam downstream to Williston, North Dakota; Figure 3.10). These latter subunits have retained a natural appearance, with a sinuous channel and a wide floodplain often with oxbow lakes, sand dunes, and interspersed patches of natural forest vegetation and agricultural fields. The natural appearance, however, masks fundamentally altered hydrologic and sediment regimes. Nonetheless, many of these subunits are not physically static, and undergo natural degradation and sedimentation processes as altered by flows and releases from upstream dams and tributary inflows. Many of these segments are now incised, which has caused the loss of adjacent wetlands and secondary channels.
Streamflow through these remnant floodplain reaches depends primarily on releases from upstream dams and secondarily on local tributary inputs. Pre- and post-regulation comparisons of streamflow through these reaches can be striking. For instance, at Bismarck, North Dakota, part of which lies on the floodplain between Garrison (upstream) and Oahe (downstream) dams, the hydrograph’s peak flow has been greatly reduced since the closure of Garrison Dam in 1953. Between 1928 and 1953, about two-thirds of the annual peak flows at Bismarck exceeded 2,500 cubic meters per second; since 1953, no peak has exceeded 2,500 cubic meters per second (Johnson, 1998). Reduced peak flows in the post-regulation period fail to inundate the floodplain, in sharp contrast to the Missouri River’s notorious floods before river regulation. Reily and Johnson (1982) showed that Garrison Dam has also changed seasonal flow patterns; peak flows now occur in winter instead of in spring, and minimum flows now occur mainly in spring and fall instead of in winter.
The lack of overbank flooding in remnant reaches, except on the lowest terraces during extreme wet periods, has serious ecological consequences. Reiley and Johnson (1982) and Johnson (1992) reported decreased rates of both tree growth and tree population recruitment due to the absence of annual recharge of water and nutrients. Moreover, the reduced post-regu-
lation peaks in Missouri River discharge have been insufficient to cause lateral meandering of the channel that is needed if recruitment sites for pioneer forest communities dominated by cottonwood and willow are to be created. This diverse community type is in serious decline in much of the Great Plains due to river regulation and land management (grazing) practices (Knopf et al., 1988).
Downstream effects on remnant floodplains are less severe below smaller, upstream dams. For example, Ramey et al. (1993) found that Canyon Ferry Dam and smaller dams on tributaries have decreased the magnitude of higher flows (those greater than 1,400 cubic meters per second) by 14-23 percent at Fort Benton, Montana. Downstream of Fort Benton, regeneration of cottonwood forests in constrained reaches (narrowed by high valley walls) depends entirely on such high flows (Auble and Scott, 1998; Scott et al., 1997). In this case, dams have reduced flooding at the expense of cottonwood forest regeneration and growth.
Water quality effects (most significantly cold water releases from middle levels or the bottom of reservoirs) are also most pronounced immediately below dams and diminish as one moves downstream. At the downstream end of remnant floodplains, streambed aggradation occurs where sediment carried by the river is dropped in the still water of the reservoir. Where aggradation is substantial and new vegetation curtails sediment redistribution, streamflow may be obstructed (Johnson, 2002). This causes flooding from above (overbank flow) and from below (rising water tables). This phenomenon has caused property damage near Running Water, South Dakota (upstream of Lewis and Clark Lake); Pierre, South Dakota (upstream of Lake Sharpe); and Bismarck, North Dakota (upstream of Oahe Reservoir).
Tributary streams within remnant floodplain reaches may ameliorate the effects of mainstem dams at specific locations (Johnson, 2002). Their influence depends on many factors, including the degree to which their flow has been regulated and their sediment trapped, their entry point on the mainstem (i.e., distance from mouth to nearest downstream reservoir), and their size (flow volume). Relatively natural tributaries contribute sediment and streamflow. The additional sediment and water contribute to higher peaks, turbidity, and greater flow variability, all of which are important to most native riverine organisms. Moreover, undammed tributaries often provide the shallow water and sandbar habitat for fish spawning and rearing destroyed by mainstem dams. Additionally, sediment input from tributaries can attenuate channel degradation below dams.
The most beneficial ecological effects of the Missouri River’s tributary streams occur when relatively large, unregulated tributaries empty into the mainstem some distance upstream of the next reservoir. The Yellowstone River is the best example of this. There are fifteen to fifty miles of the
mainstem Missouri River below the confluence, depending on the levels of Lake Sakakawea (Ryckman, 2000). The Yellowstone River adds flow and sufficient sediment to the relatively clear water released from Fort Peck Dam to cause natural cut-and-fill alluviation, riparian vegetation establishment, and successful reproduction of native fishes such as the pallid sturgeon and the paddlefish (Helfrich et al. 1999, Ryckman, 2000).
In sum, effects of reservoirs on downstream remnant floodplains are slow and progressive. These changes downstream of dams may take decades to centuries to achieve their full impacts on remnant floodplain ecosystems. However, the sediment and flow variability that tributaries contribute to some remnant reaches of the Missouri River can ameliorate some of a dam’s negative impacts. In turn, this also shortens the service period of downstream reservoirs.
Downstream of Gavins Point Dam, the Missouri River has been channelized (narrowed and deepened in a relatively fixed position) from Sioux City, Iowa to its mouth to permit navigation by boats and barges, and its banks were stabilized to enhance utilization of the bankline adjacent to the channel (sections 14-19 in Figure 3.10; Schmulbach et al., 1992). In addition, chutes and side channels have been blocked and diverted, converting the once structurally-complex channels and instream islands into a single thread of deep, fast moving water. Levees have been constructed on both banks along much of the lower river to protect crops and settlements behind them; these levees constrain overbank flows to a narrow zone of the floodplain. This channelized stretch of the river was once highly dynamic:
River surveys . . . showed that before 1930, which marked the beginning of major channel control works by the Corps of Engineers, the channel was two to three times its present width, and bars and islands existed in abundance. Floods usually brought great changes as new bars and islands were created, old ones disappeared, and the channel migrated rapidly to bring about extensive floodplain erosion and redeposition. Since 1930 and the construction of the channel control works, insular bars have appeared occasionally during low water, but their existence is only temporary; and as a consequence of channel stabilization, significant shifts of the channel even during floods have all but been eliminated (Schmudde, 1963).
The river’s upper portion in South Dakota, Nebraska and Iowa has degraded because of erosive water releases from upstream dams, the trapping of sediments in mainstem reservoirs, and insufficient flows to accomplish lateral channel readjustment. By contrast, the Missouri’s lower reaches (especially downstream of the Platte) have aggraded. In addition to degra-
dation or aggradation of the channel bed, interaction (material and energy exchange) between the river and the floodplain has been significantly reduced or totally eliminated.
Engineering works on the river’s main channel have resulted in significant ecological changes in the channelized reaches. Construction of revetments has greatly narrowed and deepened the channel and has fixed its location. This has virtually eliminated shallow water habitat and greatly increased water depth and current velocity. Ecological impacts of these changes on native fish and on streamside vegetation have been strongly negative (Schmulbach et al., 1992). Levees restrict the river to only a small portion of its total floodplain, except during rare floods such as in the 1990s, when some levees were breached and water and sediment moved behind the levees into floodplains (Galat et al., 1998). Overall, the levee system has reduced interaction between the river channel and its floodplain, resulting in the inability of the river to sustain its historic levels of biodiversity.
MISSOURI RIVER ECOSYSTEM SCIENCE
The Missouri River ecosystem has been the subject of scientific investigations that date back to Meriwether Lewis and William Clark and their cataloguing of Missouri River plant and animal species during their epic journey of 1804-1806. The pace of scientific investigations of the ecosystem increased notably during the second half of the twentieth century. A recent bibliography of technical reports and scientific investigations lists well over two thousand entries prior to 1997 (Burke et al., 1997). At least several hundred scientific publications have been added between 1997 and the publication date of this report.
A comprehensive review and analysis of that entire body of science was beyond this committee’s means and scope. Furthermore, that research is unevenly distributed across topics; for example, there has been more scientific inquiry into select species, such as those on the federal endangered species list, than into other ecological topics such as carbon cycling or plant and animal interactions. In addressing its charge to identify the general state of that information, and to identify and prioritize key scientific questions and information, the committee thus focused its reviews on the two ecosystem components that have received the bulk of scientific attention, fisheries and floodplain vegetation.
Research on Fisheries
Ecological impacts of large mainstem dams and other human activities in the Missouri River basin were slow to be discovered. The river’s biota
had been at least superficially inventoried between the mid-nineteenth and twentieth centuries (e.g., Aikman, 1929; Allen, 1875; Bailey and Allum, 1962; Bennett, 1931; Fisher, 1945; Gilmore, 1911; Jordan and Meek, 1885; Linsdale, 1928; Perisho and Visher, 1912; Reid and Gannon, 1927; Stevens, 1945). However, connections between key physical processes and key ecological processes remained virtually unstudied for the Missouri River, and most large rivers, until late in the twentieth century (Hesse et al., 1989; Stanford et al., 1996). Detecting change itself during this period was difficult because the Missouri River’s baseline conditions were only partially known. Moreover, when changes were documented, the causes were unclear because of the increasingly complex mix of human and natural factors affecting the river ecosystem.
Among the better-documented ecological changes on the Missouri are the development of sport and recreational fisheries in the large mainstem reservoirs, especially in the three largest reservoirs—Lake Sakakawea (Garrison Dam), Lake Oahe (Oahe Dam), and Fort Peck Lake (Fort Peck Dam). The clear water in the reservoirs provided an advantage to “sight feeding” native species, such as the walleye, which was a species in relatively low abundance whose numbers increased dramatically with habitat changes caused by the reservoirs. Just as these environmental changes made conditions better for some species, other species that were better adapted to pre-regulation conditions, such as the sauger, experienced declines with the replacement of a free-flowing river by the system of reservoirs. The key introduced sport species on the Missouri River is the chinook salmon. Rainbow smelt and spottail shiner have also become established and are a major food source for the salmon and the walleye. Northern pike numbers increased dramatically with construction of the reservoirs. Once the dominant sport species immediately following construction of the reservoir system, the northern pike has declined in numbers and today represents a relatively insignificant portion of the sport catch. White and black crappies responded well to the filling reservoirs and became major panfish species for a few years. The shovelnose and the pallid sturgeon are among the native species that have nearly disappeared in the reservoirs. The paddlefish has also been extirpated from much of the reservoir system, with remnant populations above Fort Peck Lake, at the confluence of the Yellowstone and Missouri rivers, and near the mouth of the Niobrara River.
Symptomatic of the changes that have occurred in the Missouri River and floodplain ecosystem are the appearance of three federally listed threatened or endangered species. These are the least tern (Sterna antillarum), piping plover (Charadrius melodus), and a unique fish species, the pallid sturgeon (Scaphirhynchus albus). These species have generated much at-
tention with respect to prospective changes in Missouri River dam and reservoir operations, as the Corps of Engineers must respond to jeopardy biological opinions issued by the U.S. Fish and Wildlife Service regarding dam operations and the continued existence of these species (Appendix A, Table 4 lists the fish species found along the mainstem of the Missouri River today. The appendix also includes fishes that may exist on the floodplain in small creeks, or in overflow pools and oxbow lakes).
The Pallid Sturgeon
The pallid sturgeon was listed as endangered throughout its entire range on September 6, 1990, and the species is currently considered close to extinction (Dryer and Sandvol, 1993). Pallid sturgeons were thought to live primarily in large, turbid rivers such as the Missouri, and the Mississippi River downstream from its confluence with the Missouri. It utilized overflow areas on the floodplain, backwaters, chutes, sloughs, islands, sandbars, and main channel banklines, pools, and snags (Dryer and Sandvol, 1993). Because it feeds on aquatic invertebrates and fish that prey upon aquatic invertebrates, the lower velocity margins of the main and extra channels were essential habitats for the pallid sturgeon (Carlson et al., 1985). Some information suggested that pallid sturgeon readily utilized off-channel habitats for feeding and nursery and main channels for spawning (Dryer and Sandvol, 1993; Keenlyne, 1989; Zuerlein, 1992). Some scientists have expressed concern that pallid sturgeon cannot reproduce in the Missouri River’s channelized and reservoir habitats (Henry and Ruelle, 1992; Ruelle and Henry, 1994). In 1993 it was concluded that sturgeon populations will continue to decline and that riverine habitat alteration and destruction are negatively impacting sturgeon recovery (National Paddlefish and Sturgeon Steering Committee, 1993).
Plains Minnows and Sauger
The plains minnow and the sauger are two examples of common Missouri River native fish species that declined rapidly in the aftermath of dam construction and channelization. Plains minnows were once considered the most abundant minnow in the portion of the Missouri River in upper Missouri (Cross, 1967; Fisher, 1962; Jones, 1963; Morris et al., 1972; Pflieger, 1975). This small minnow was well adapted to the river’s turbid environment. It lived among the numerous sandbars, feeding on living and dead plant material, and was an important component of the trophic web of the pre-regulation Missouri River (Hesse, 1994). The plains minnow has experienced a dramatic decline in abundance and is a much smaller component of the species composition today (Figure 3.11). It rebounded for a few
years during a wet period between 1993 and 1998, but the increase in abundance was quickly reversed, as floodplain connectivity was severed during 1998 and 1999.
Saugers were common prior to channelization and impoundment of the Missouri River (Cross, 1967; Jones, 1963; Jordan and Evermann, 1969). The species comprised between 10 and 65 percent of the main channel large-river fish assemblage. They have since declined by as much as 98 percent in some locations in the river (Figures 3.11 and 3.12; from Hesse, 1994). Sauger were important sport fish of exceptional food quality, and recreational anglers fished for sauger before the mainstem dams were built. They are closely related to the walleye except they were widely adapted to the turbid environment of the Missouri River, and they were much more numerous than walleye before river regulation.
The list of threatened or endangered Missouri River species continues to grow. Whitmore and Keenlyne (1990) noted that 82 species found along the Missouri River were listed as rare, threatened, or endangered by the seven states bordering the river. Included were 24 fish, 22 birds, 14 plants,
8 reptiles, 6 mammals, 6 insects, and 2 freshwater mussels (Appendix B lists threatened and endangered species along the Missouri River).
Research on Floodplain Vegetation
Prior to twentieth century human-induced environmental changes, the Missouri River’s floodplain vegetation was a storehouse of biodiversity. One of the few comprehensive surveys of the floodplain forest flora found 220 species of vascular plants growing in the remnant river section between Garrison Dam and Oahe Reservoir in North Dakota (Keammerer et al., 1975). This inventory was conducted long after extensive forest clearing had occurred and did not include a comparably rich flora of wetland plants found in non-forest communities on the floodplain. Studies of this 75-mile floodplain remnant by Keammerer et al. (1975) and Johnson et al. (1976) revealed a mosaic of aquatic, riparian, and terrestrial communities, including oxbow lakes, ponds, marshes, sand dunes, shorelines, in-channel islands, sand bars, forests, and agricultural fields.
Natural vegetation communities along the Missouri featured forests with a wide variety of species. The dominant floodplain trees were cottonwood (Populus deltoides), green ash (Fraxinus pennsylvanica var. lanceolata), box elder (Acer negundo), and American elm (Ulmus americana). Subdominant trees included peach-leaved willow (Salix amygdaloides) and bur oak (Quercus macrocarpa). Common shrubs and woody vines included dogwood (Cornus stolonifera) wolfberry (Symphoricarpos occidentalis), poison ivy (Rhus radicans), chokecherry (Prunus virginiana), juneberry (Amelanchier alnifolia), woodbine (Parthenocissus inserta), and fox grape (Vitis vulpina).
Johnson et al. (1976) determined that these forests formed a successional series of ecological communities, from the youngest—dominated by cottonwood-willow formed on fresh alluvium on low benches—to the oldest—dominated by ash-box elder-elm on high benches. The river initiated the succession by meandering across its floodplain during floods and eroding older forests on the outside of river curves while creating point bars on the inside of curves for pioneer tree establishment. Approximately two-thirds of the floodplain forest flora occurred in the successional cottonwood forests that depend on river meandering.
No equally comprehensive inventories of floodplain vegetation have been published in downstream sections of the river; however, general descriptions are available and general ecological relationships are known. Downstream from North Dakota, the overall floodplain flora becomes considerably richer, particularly the woody component. For example, twenty-one species of trees were found on the Missouri River floodplain between Sioux City, Iowa and Rulo, Nebraska (Vaubel, 1973), approximately three times more than along the Missouri River in central North Dakota (Johnson et al., 1976).
On the lower portions of the Missouri River, cottonwood and willow were the dominant species on recently deposited and exposed sandbars, as they were throughout the length of the Missouri (Galat et al., 1996; Hesse et al., 1988). Later successional species were more diverse than in northern reaches of the river. For example, box elder, silver maple (Acer saccharinum), red mulberry (Morus rubra), and several elms replaced cottonwood and willow and formed an intermediate successional stage. The mature forest included several species of oaks (Quercus spp.), hickories (Carya spp.), black walnut (Juglans nigra), basswood (Tilia americana), hackberry (Celtis spp.), and sycamore (Platanus occidentalis). Weaver (1960) reported that along the Nebraska section of the Missouri River, forest, shrubs, and coarse grasses occupied most of the active floodplain, while higher terraces were nearly covered with prairie.
Much of this diverse and extensive floodplain forest was cleared before significant regulation of the river. A large portion of these woodlands was
removed to provide fuel for steamboats during the nineteenth century, and more recently for agriculture. In the section of the Missouri River in North Dakota between Garrison Dam and Oahe Reservoir, 38 percent of the floodplain forest was cleared for agriculture between 1881-1938, an average rate of approximately 0.7 percent per year (Johnson et al., 1982). An additional 18 percent of the forest was cleared for cultivation between 1938-1979, an average rate of approximately 0.5 percent per year. The majority of the woodlands along the river were removed to provide fuel for steam-powered vessels during the nineteenth century, and more recently for agricultural purposes. Thus, clearing activities claimed approximately 56 percent of the original floodplain forest by 1979. Considerably more forest has been cleared since then, but the amount has not been quantified.
Clearing of the Missouri’s floodplain forest occurred much earlier along the river’s lower portions. Bragg and Tatschl (1977) estimated that 76 percent of the floodplain in Missouri was forested in 1826. By 1937, the percentage had dwindled to 17 percent, and by 1972 had dropped to 13 percent. The percentage of the floodplain cultivated increased accordingly, from 18 percent in 1826 to 83 percent in 1972.
Hesse et al. (1988) estimated cover changes on the floodplain for a larger area, from St. Louis upstream to Rulo, Nebraska, including the area studied by Bragg and Tatschl in 1977. Cultivated land increased 43-fold between 1892 and 1982 (from 2,339 hectares to 100,091 hectares), while during the same period forests decreased by 41 percent, wetlands by 39 percent, sandbars by 97 percent, and grasslands by 12 percent (Hesse et al., 1988). The authors suggested that higher floodplain ground was cleared of forest prior to the dams. Dam construction and the cessation of flooding then stimulated another round of forest conversion to agriculture downstream of and between the reservoirs, this time on lower ground.
Significant changes in Missouri River fauna also occurred long before river regulation. For example, some of the nearly 160 species and their habitats first described by Lewis and Clark (Burroughs, 1961) went extinct or were extirpated from the region because of hunting and loss of habitat. These species included bison, elk, grizzly bear, wild sheep, swans, and the Carolina parakeet. These animals used the Missouri River valley as protection from harsh winters and summer heat. The beaver was essentially trapped out of much of the northern Great Plains. Wetland drainage, stream channelization, and trapping kept beaver numbers low during much of the first half of the 1900s (Bennitt and Nagel, 1937). More recently, due partly to the declining value of wild fur, beaver are more numerous even along portions of the channelized reaches (Larry Hesse, River Ecosystems, Inc., personal communication, 2001).
Botanical research from 1960 to 2000 on the upper Missouri River centered on the effect of dams on floodplain forest succession, particularly
the effect of altered hydrology and sediment regime on river meandering and on cottonwood regeneration. Natural succession patterns were worked out in remnant reaches shortly after the dams were built. No comprehensive studies of forest succession were conducted prior to construction of the mainstem dams. Botanical research was concentrated in two Missouri River reaches—the unchannelized floodplain downstream from Gavins Point dam (Wilson, 1970), and the remnant floodplain between Garrison Dam and the headwaters of Oahe reservoir, near Bismarck, North Dakota (Johnson et al., 1976). Later investigations of cottonwood regeneration in the reach of the Missouri River’s section designated as a national monument in Montana (formerly designated Wild and Scenic) were conducted by Scott et al. (1997) and others.
The studies in North Dakota reconstructed (post hoc) the patterns and processes of forest succession on the Missouri River floodplain under natural, pre-regulation conditions, concluding that the key geomorphic process directing vegetation succession was the river’s meandering channel. The Missouri River moved across its floodplain during high flow periods; its rate of movement was especially rapid during floods. In places, the river had moved several miles in less than a century (Johnson, 1992). During episodes of meandering, the outer bank of the river eroded while the inner bank accreted. On the river’s outer curve, forests in various stages of successional development, along with other floodplain habitats such as sand dunes and marshes, were undermined. On the river’s inner curve, new land formed that was ideal for forest regeneration. During floods, the Missouri River channel was filled with fallen trees, which eventually settled to the bottom of the river and became snags. Cottonwood and willow (both tree and shrub growth forms) were the first woody species to colonize the newly accreted areas (point bars). They dominated the pioneer vegetation throughout the length of the Missouri River, but were especially important in the upper, more westerly, reaches where few other tree species could grow.
The success of cottonwood and willow was attributed to their specific adaptations to riparian zones, which includes: rapid seed germination to grow immediately after spring floods, rapid root and height growth enabling tolerance to flooding, drought, and sedimentation, tolerance to the often low soil fertility on sandbars, and the ability to reproduce vegetatively, particularly after physical damage from floods or from beaver. Evidence for the success of cottonwood was its historic dominance of the floodplain vegetation; over eighty percent of the extensive forests on the floodplain of the pre-regulation Missouri River in North Dakota had cottonwood as their most important tree (Johnson et al., 1992).
Research also uncovered a key fact about cottonwood: It cannot reproduce successfully in its own forests. As such, it depended on the creation of
new land from an actively meandering river for its persistence and prominence in the vegetation. It behaved as a disturbance-dependent, fugitive species that declined in abundance in stable portions of the floodplain while increasing in other portions recently reworked by the Missouri River. Later successional tree species replaced it over time in stable areas.
The Missouri River’s pre-regulation floodplain was a mosaic of forests with a wide range of ages, from young cottonwood-willow forests a decade or two old, to forests of ash, box elder, elm, and oak that were old enough to have lost all traces of the cottonwood pioneer element. Of the forests studied by Johnson et al. (1976), approximately two-thirds were early to mid-successional (dominated by cottonwood), while one-third were dominated by later successional tree species.
High biodiversity both within forest communities and across the floodplain could not be maintained without the rejuvenating forces of floods and channel meandering. Johnson et al. (1976) found the highest tree diversity mid-way through succession, a period when all tree species grew together, with cottonwood and willow in the overstory, and with ash, box elder, elm, and oak in the understory. Hibbard (1972) found similar patterns for forest-dwelling animals; the number of species of both small mammals and birds peaked in forests of mid-successional age. This ephemeral, species-rich stage was historically sustained by new forests created by a meandering river.
Hibbard (1972) found that the floodplain forest community provided nesting habitat for a wide range of bird species, from open country birds in the youngest, post-flood communities, to shrub-loving bird species in middle-aged cottonwood communities, to forest-dwelling birds in the most mature forests. Peaks in both species number and population of birds were reached in older successional forests because of their high vertical stratification (Hibbard, 1972). The large size and hollow trunks and branches of older cottonwood trees provided nesting cavities for woodpeckers and wrens (Knopf, 1986). More than 50 species of songbirds were found by Liknes et al. (1994) along the upper Missouri River, and approximately 50 percent were neotropical migrants. Dean (1998) found 39 species of neotropical migrants utilizing Missouri River floodplain forests as stopover habitat.
Model calculations suggest that without changes to the current management regime, cottonwood forests will essentially be lost as a significant community on remnant floodplains in less than a century (Johnson, 1992). The cottonwood forests that remain on the floodplain between and immediately below dams cannot be sustained by the current low river meandering rates. Both erosion and deposition rates (an index of river meandering rate) have decreased substantially since the closure of the mainstream dams. Deposition rates have fallen to one percent of their pre-regulation levels and erosion rates have fallen to twenty-five percent of their pre-regulation lev-
els. Cottonwood forests are forecast to be replaced by those dominated by green ash. These future forests, assuming that they escape clearing for agricultural expansion, are likely to be considerably lower in tree and bird diversity primarily because of the loss of pioneer plant species, loss of vertical structural complexity, and the loss of nesting cavities found mostly in old cottonwood trees.
Reduced channel meandering was not the only impact of flow regulation by dams on floodplain vegetation. Reily and Johnson (1982) found that seasonal shifts in flow, the near-elimination of overbank flooding, and a lowering of the water table below the floodplain in late spring, reduced the growth of trees occupying remnant floodplains between reservoirs. Peak river flow no longer occurs early in the growing season. It is consequently out of phase with the vernal growth pattern typical of floodplain trees. The absence of flooding except on the lowest benches represents a loss of soil moisture to the floodplain compared to pre-regulation conditions. Climatic conditions in the upper Missouri River region are relatively dry. As such, overbank flooding was important for the growth and persistence of trees.
Several scientists have recommended measures to regenerate cottonwood forests in sections of the Missouri River affected by mainstem dams. Johnson (1992), for example, suggested planting native pioneer trees, such as cottonwoods and willows, on marginal agricultural land unless a more natural flow regime could be restored. While planting can, with time, maintain certain important ecosystem properties, such as cavity-nesting habitat for birds, it cannot restore certain other properties, such as the high species diversity of early successional forests. Pre-regulation forests were established on relatively low benches and were repeatedly aggraded by siltation from floods. As a result, these communities supported a significant proportion of wetland species. This species diversity cannot be restored by tree planting on relatively high benches.
The Missouri River ecosystem experienced a variety of human-caused environmental changes during the twentieth century. Before these changes, the Missouri River experienced annual floods, with occasional massive flooding. It meandered freely across its floodplain, it carried large amounts of debris and snags, especially during floods, and it eroded, transported, and deposited voluminous amounts of sediment. These dynamic geomorphic processes promoted erosion on the river’s undercutting banks and deposition on its inner banks. This pre-regulation, physical variability was important to sustaining the river system’s biological diversity and production.
Engineering works constructed during the twentieth century aimed to enhance conditions for navigation and to provide protection against floods by reducing this variability. Mainstem dams and channelization greatly changed the river’s physical systems. Much of the river’s huge sediment load was deposited in the massive reservoirs, resulting in sediment imbalances and marked channel incision below the dams. The massive mainstem reservoirs submerged stretches of free-flowing river and floodplain forest. Changes in habitat allowed some native species that were challenged by pre-regulation conditions to thrive. Some species that thrived under pre-regulation conditions, such as the pallid sturgeon and sauger, experienced sharp reductions. The river’s periodic flooding also was greatly reduced, and even eliminated, in stretches under the dams’ stabilizing influences. This resulted in the reduction or loss of ecologically-beneficial flood pulses and low flows. The hydrologic connections between the river channel, floodplain, and backwater areas were greatly disrupted. In the channelized portions, river meandering was eliminated and the ecological diversity of the river and floodplain was greatly simplified.
A large body of scientific research has identified these ecological changes, including declines in many native species and a general decline in the overall integrity of the ecosystem that have accompanied the mainstem dams and other human influences on the ecosystem. As in all large ecosystems, uncertainties—some of which are essentially irreducible—remain in the scientific understanding of the Missouri River and floodplain ecosystem. Nonetheless, the scientific research provides a good picture of the fundamentals of the ecosystem’s pre- and post-regulation ecological structure and function.
Although much contemporary research retains a species-specific focus, some scientists and organizations are integrating research across disciplines and across the entire river system. This broader perspective and inquiry will complement the existing science in promoting a more systematic understanding of the Missouri River ecosystem.
Mainstem dams and reservoirs, channelization, and a management regime that promotes hydrologic stability have all contributed to reductions in the ecosystem’s dynamic properties. Current scientific inquiry is thus hindered in its ability to investigate these dynamic processes and how they affect the ecosystem and its cadre of organisms. The greatest unknowns in the science of this ecosystem are in understanding its responses to changes in the current management regime.