Sediment Management Alternatives and Opportunities
The Corps of Engineers Missouri River emergent sandbar and shallow water habitat construction programs described in Chapter 4 include excavation and disposal of sediment within and near the Missouri River’s main channel. Other sediment management activities have also been proposed, some of which are being implemented locally, but not yet extensively along the river. In addition, questions have been raised recently about the potential for sediment management along the Missouri to affect coastal wetlands and marine water quality as far away as the Mississippi delta of Louisiana and the hypoxic zone in the northern Gulf of Mexico.
Some of the proposed sediment management activities are technically feasible, while others are more complex, uncertain, expensive, and as yet are subjects only of preliminary discussion and analysis. Manipulation of sediment along a river that has already been significantly engineered (Chapter 2) raises many questions about intended or unintended consequences, effectiveness, and time scales of expectable responses.
This chapter explores the primary alternatives for reintroducing sediment into the Missouri River. Specifically, the chapter answers, but is not limited to, the following questions in the statement of task:
Question 6: Are there alternatives for reintroducing sediment into the system? What are they and what are the key constraints surrounding these alternatives?
Question 3: What is the significance of Missouri River sediments to the restoration of Louisiana’s coastal wetlands?
The alternatives explored most extensively in the chapter include habitat construction, removal of riverbank control structures, limitations on commercial dredging, bypassing of sediment around mainstem dams, dam removal, and increasing sediment deliveries from tributaries. All of the discussions are exploratory and aimed at definition of the problem and its implications. None of the discussions should be construed as recommendations for or against any course of action. Decisions about particular actions would require more extensive analysis and will depend on future development of economic, engineering, and environmental conditions along the Missouri River.
MISSOURI RIVER SEDIMENT REINTRODUCTION ALTERNATIVES
Emergent Sandbar and Shallow Water Habitat Projects
The Corps’ Emergent Sandbar Habitat and Shallow Water Habitat projects have implications for sediment loadings and transport and therefore for channel morphology and habitat maintenance. Sediment in transient storage during its passage along the river channels and floodplains of the Missouri River valley has value for habitat formation and has both positive and negative influences on infrastructure. As discussed in Chapter 2, these implications are in addition to the direct ecological dimensions of the projects described in Chapter 4.
The Corps of Engineers currently is constructing emergent sandbar habitat in the approximately 40 miles of river channel between Gavins Point Dam and Ponca State Park (see Figure 4-1). The bed of the Missouri River in this reach has been degraded (scoured deeper) by as much as 12 feet since dam closure (Jacobson et al., 2009). Sandbars in this reach of the river usually are constructed from sand dredged from the channel bed, and therefore likely to consist largely of relatively coarse, slow-moving bedload sand. The constructed bars gradually erode, however, and their sand is redistributed to the bed with no net effect on the river’s sediment balance. These sandbars need to be replenished every few years.
The Corps also is constructing shallow water habitat in and along the lower Missouri River channel downstream of Ponca State Park (Figure 4-1). At some sites, chute and backwater channels are excavated in the floodplain (sometimes taking advantage of former natural chutes) and the sediment is returned to the main channel. The sediments are fed into higher-velocity areas of the river and are thus dispersed downstream. An associated strategy involves excavating sediment to depths of several feet along the margins of the main channel and building structures to slow the flow along some channel margins. Material from the main-channel margin
is likely to be coarser than sediment from chute excavations. Because of the stratified nature of typical floodplain sediments, it may be possible to separate excavated material dominated by chemically active clays and silts from more inert sandy deposits, and then selectively return only the sandy deposits to the channel.
A 2009 report authored by U.S. Geological Survey and Corps of Engineers professional staff contains the most valuable data set available for a quantitative assessment of the impacts of this practice on the sediment budget and water quality (Jacobson et al., 2009). That report describes how, according to current habitat construction plans, approximately 15,000 acres of land that accumulated in the decades following Bank Stabilization and Navigation Project implementation would be excavated, and approximately 34 million tons (MT)/year of sediment would be returned to the channel in each of 15 years. This would amount to an increase of about 60 percent of the current annual sediment load passing Hermann, Missouri.
Measurements of grain-size distributions of excavated material at sampling sites at Jameson Island (Figure 5-1) indicate that if the excavated spoil is transported quickly through the narrowed and leveed mainstem channel (which is designed to keep the sediment in motion), the sand load passing Hermann would be almost tripled—from 14 to 40 MT/yr—and the silt-clay load would be increased by about 20 percent, from 41 to 49 MT/yr (Jacobson et al., 2009). The silt-clay load would travel as washload, enhancing turbidity and traveling rapidly downstream with only small amounts being trapped in off-channel shallow water habitats. The sand, having formerly entered the floodplain mainly as suspendible bed-material load (Chapter 2), would be flushed downstream and dispersed as transient additions to bars and floodplains along the lower Missouri River. Some of the sand excavated from the floodplain would be coarse enough to travel as slowly moving bedload, but this component is likely to be small, especially in the reach upstream of the Platte River where the bed has degraded by as much as 12 feet since dam closure.
Downstream of the confluence with the Platte River, Missouri River bed degradation is locally variable or absent (Jacobson et al., 2009), and an increased sand load of the magnitude proposed under full project implementation requires careful consideration of its potential for causing some bed elevation, bar building, and complications for navigation. The problem could be assessed in greater detail through a detailed modeling study, utilizing measured grain-size distributions and channel morphology, of the probable impact of the temporary sand storage on potential cross-section changes. Existing data for the river could be used for such an assessment, and model results could be evaluated with high-value monitoring resources of relevant federal agencies. Such a study also would allow consideration of
lengthening the habitat construction period if the capacity of the channel was inadequate to pass the sand without bed-elevation changes.
Removing Bank Stabilization Structures
Downstream of Sioux City, Iowa, the Missouri River is channelized and controlled by levees, revetments, jetties, wing dikes, and other river training and control structures of the BSNP (USACE, 2009b). These structures make it difficult to establish and maintain complex channel and floodplain habitats that depend on the temporary storage and frequent alteration of sediment accumulations in the form of channel bars and off-channel water
bodies. The control structures are designed to maintain a narrower and more rapid flow that scours a deeper and less complex channel. The degree to which economic activity, transportation, and public safety along the Missouri River depend on these control structures makes it unlikely that the river ever will be extensively released from these constraints. However, the purchase by the Corps of Engineers of some leveed, but still flood-prone, land along the Missouri River raises the possibility that over coming decades some of these river control works may be breached in rural reaches, either naturally or by design. Lands acquired through September 2009 under the Missouri River Mitigation Program totaled 56,606 acres, or 34 percent, of the 166,750 acres authorized for purchase (Bell and Bitner, 2010). Large areas of other flood-prone land are protected by nonfederal levees that are aging and that place maintenance burdens on local levee districts.
The river will migrate laterally if revetments are not maintained. In some reaches, such migration would undermine sediment that was stored during the era of bank stabilization, causing a net increase—or more exactly a partial restoration—of the river’s sediment load. If so, the general effects on sediment transport, sedimentation, and water quality would be similar to those discussed above for chute excavation. The magnitude and location of individual impacts of this kind are beyond the scope of this report, but could be more carefully evaluated if resources were applied to the problem. Since such a program of releasing the river from its lateral constraints would happen gradually over many decades, and would be accompanied by both sediment removal from the floodplain and sequestration of sediment on the floodplain, the net effect on the downstream annual sediment flux would not be a large percentage change. Over coming decades, however, the sediment locked in storage by Bank Stabilization and Navigation Project structures would be released gradually back into the river.
It is not likely that reconfiguring the navigation channel will be either desirable or acceptable to the many communities, farms, and other parties along the Missouri River in the near future. These parties have legitimate concerns about the potential impacts of channel reconfiguration (e.g., widening of the channel for the purpose of creating extensive shallow-water habitat and sandbars within and along the margins of the channel) or changes in river bed elevation. These changes could affect erosion and flooding, with consequent impacts on homes, infrastructure (transportation, telecommunications, power), farmland, and other floodplain property. At some point in the future, changes in technology or economics may decrease the importance of maintaining a large, viable channel for commercial navigation. At that time, the potential might become more attractive for freeing the channel to migrate within suitable public lands or private lands with flood easements and for increasing the width of the active floodplain subject to regular inundation.
In the interest of reviewing all potential consequences of channel widening to the extent possible, reestablishing some of the river’s wide, shallow, braided channel characteristics in any of the river’s reaches would benefit river ecology by allowing more space and more natural sediment storage and transmission processes for the creation of shallow-water and emergent-sandbar habitat. A number of previous studies have documented the potential benefits of restoration of some of the prehistoric inundation and alluvial processes (Galat et al., 1998; NRC, 2002; Opperman et al., 2010). Creating habitat in the vicinity of the present navigation channel is difficult because the narrow channel inhibits the sediment accumulation needed for the creation of sandbars. It is also difficult to make river-wide generalizations about the magnitude and sustainability of simultaneously maintaining navigation and reconfiguring the channel for ecological benefits without a thorough, reach-by-reach assessment and modeling study.
Smaller programs of levee setback and opening of secondary channels have been executed on large alluvial rivers in Europe, including the lower Rhine (Buisje et al., 2002), the Danube (Schiemer at al., 1999), the Loire (Belleudy, 2000), and the Rhone (Amoros, 2001). Predicting the impacts of major channel reconfiguration in the Missouri is difficult because no similar projects have been attempted there. There are no river restoration projects in the United States that have removed channel controls on the scale of the Missouri River, and such a program would have to evolve gradually in the context of regional development so as to minimize possible impacts and disruptions to other sectors.
Several more limited and local strategies, applied singly or in combination, could be used to move the channel toward a more complex and dynamic configuration, with more secondary channels, sandbars, and shallow water habitat: Removal of revetments and removal or reshaping of dikes would allow the river to erode its banks, resulting in widening of the channel. Assuming that sediment was available (which in the short term is likely to be true only downstream of the Platte River confluence), this process would likely lead to reestablishment of sandbars and development of shallow water habitat at the margins of the main channel, as has been observed in side channels without revetments (Jacobson et al., 2004b). Levees could be set back, or aging levees could be allowed to fail naturally, to allow floodwater access to portions of the floodplain and the river to reoccupy and scour out some inactive side channels during particularly high flows. Riverside sites with major infrastructure investments, such as settlement, transportation, water supply and power plants, would continue to require protection with existing or enhanced revetments and levees. The Corps has been experimenting with local projects to notch and otherwise modify dikes since the 1970s. Given current uncertainties, the process of gradual
channel reconfiguration on a local scale could continue and be conducted as part of an adaptive management strategy. That is, appropriate monitoring could be used to support additional fluvial geomorpohology modeling and hypothesis testing, broaden the options explored, and anticipate likely outcomes based on interpretations of floodplain sedimentology, river channel mechanics, and ecological linkages.
Limitations on Commercial Dredging
The lower Missouri River has been dredged since the nineteenth century to permit navigation. Since completion of the 9-foot channel and following closure of the dams upstream in the early 1960s, much less dredging has been required. During the last several decades, however, volumes of commercial dredging on the mainstem and its tributaries increased from approximately 1.3 MT/yr in 1974 to 8 MT/yr in 2006 (USACE, 2008). Most commercial dredging on the mainstem is concentrated near the larger urban centers, particularly Kansas City. Dredging in the Missouri River’s tributaries is not well quantified. However, roughly 1.4 MT/yr of sand are dredged from the lower Kansas River, contributing to bed degradation and related infrastructure problems (e.g., degradation of soil under bridge foundations) in the lower Missouri River (Rasmussen et al., 2005). Dredging and removal of sandy bed-material load from the major tributaries contribute directly to reduced sediment loads in the mainstem Missouri River and at points downstream. In principle at least, reducing sand dredging from the reach downstream of the main right-bank tributaries will ameliorate channel-bed degradation in the Missouri River near Kansas City.
It is estimated that commercial dredging between 2003 and 2005 removed an amount equivalent to about 40 percent of the sand flux during the same period past Hermann, Missouri (Jacobson et al., 2009). However, quantification of the likely impact on bed elevation requires a model-based estimate of bed-material transport and storage. Given the simplicity of the channel and the amount of existing data on sediment characteristics, hydraulics, and channel geometry within the lower Missouri River, such an analysis would be quite straightforward.
Bypassing Sediment Around Mainstem Dams
Bypassing of sediment around major dams has been suggested as a means of ameliorating some of the undesirable effects of the massive reduction of sediment flux down the Missouri River since the 1930s. The potential for substantial sediment releases from these dams would depend on (1) volumes and grain sizes of sediment that can be mobilized, (2) the rate at which each grain-size component could travel downstream, and (3) the
degree to which each grain-size fraction would be stored in the floodplain along the Missouri and Mississippi rivers.
The most important and implacable of these constraints lies in the fact that most of the sediment that has been stored behind dams in the Missouri and its tributary basins is far upstream of any direct connection to the Mississippi and even to the lower Missouri River. This discussion thus focuses on the lowermost dam, Gavins Point, because if bypassing of sediment is to become feasible, it is most available at this most downstream impoundment. It must be emphasized, however, that all experience with sediment bypassing has been accumulated on dams much smaller than those on the Missouri, and investigations of what is possible for large dams remains within the realm of mathematical modeling.
There are two fundamental strategies for moving sediment past reservoirs and dams: capturing and diverting sediment before it deposits in the reservoir, or remobilizing sediment that has accumulated within the reservoir. The first involves constructing a canal or pipeline to collect and convey sediment over the dam or through low-level outlet works; this is the only option in very large reservoirs. For example, sediment bypassing opportunities have been designed into Chinese dams at Three Gorges on the Yangtze River and at Xiaolangdi on the Yellow River, and they have been retrofitted into the rock wall of the canyon at the older Sanmenxia dam on the Yellow River. The general principle behind the design of these works is to open the dam flow control structures when sediment-rich flows are expected and allow the river to flow freely through the reservoir and bypassing works. The effectiveness of their operations has not yet been widely documented or analyzed. An alternative is remobilization of already-deposited sediment in the reservoir and moving it past the dam using flushing, sluicing, or hydraulic/hydrosuction dredging, and discharging it into the river downstream.
Sediment flushing is conducted by draining the reservoir through low-level outlets and allowing river-like conditions to be established throughout the sediment deposits. The increased water velocity mobilizes deposited sediment and moves it downstream. Flushing is practiced worldwide and guidelines for its implementation have been published (e.g., White, 2001). The method is performed in two reservoirs within the Missouri River basin: Guernsey Reservoir on the North Platte River, and Spencer Dam on the Niobrara River. Sediment sluicing is similar to flushing except that the reservoir is only partially drawn down during the operation, leaving some water storage available upstream. The increased water velocities and tractive forces mobilize only sediment deposited near the dam. Hydraulic and hydrosuction dredging techniques transport sediment-laden water via pipeline from the reservoir to the downstream river. Hydraulic dredging requires an externally powered pump, while hydrosuction dredging uses the suction pressure generated by the elevation differences between the up- and
downstream water levels to drive the sediment and water into the pipeline (Hotchkiss and Huang, 1995).
The decision on the most appropriate method for moving sediment past a dam and making the dam and its reservoir sediment neutral for the river depends on economic analyses of costs and benefits, the acceptability of the method and its results to stakeholders, and on laws and institutional agreements for Missouri River operations. It might also include relief from negative impacts in the reservoir itself, such as the reduction of operating efficiency and the raising of riparian ground water levels around the accumulating wedges of sediment. The physical factors that influence the decision are largely the size of the reservoir with respect to the amount of entering sediment and the size of the reservoir with respect to the through-flow of water (Coker et al., 2009). A reservoir that is small with respect to the amount of sediment and water flowing into it may be a reasonable candidate for sediment flushing, whereas a reservoir that is large in comparison to its sediment and water inflows may be better managed through sluicing.
Gavins Point Dam and its reservoir, Lewis and Clark Lake, represent an example of a system where flushing is likely to be the best solution for moving sediment past the dam (Coker et al., 2009). Lewis and Clark Lake is relatively small in comparison to the water and sediment that enter its upstream areas, and draining the reservoir through low-level outlets would likely hasten the movement of sediment from the lake to downstream areas. Some of the sediments entering Lewis and Clark Lake from the Niobrara River have already been flushed through a reservoir at Spencer Dam.
It has been estimated that about 6 MT/yr of sediment currently accumulate in the lowermost reservoir behind Gavins Point Dam (Coker et al., 2009). That estimate envisioned the eventual (50 years from now) passing of sediment around Gavins Point Dam at the rate of 6 MT/yr. Most of the sandy sediment entering this reservoir is stored at the upstream end of the Niobrara River delta in Lewis and Clark Lake. There are severe constraints on bypassing coarse sandy sediment, and one proposed bypassing strategy envisions using much of the current stored sand to raise the elevation of the tributary deltas rather than bypassing it around the dam (Coker et al., 2009). In the same report, it was estimated that 60 percent of the released sediment would be silt and clay and approximately 25 percent of the sand would be fine enough to behave as washload through the degraded reach (estimated from figures in Coker et al., 2009). Therefore this action will result in little if any sediment settling to the bed of the lower Missouri River below the dam to ameliorate the bed degradation.
Almost all the bypassed sediment would be flushed quickly downstream, with the bed-material sand participating in transient storage bars within and along the margins of the channel. This bar-building material is likely to reside for longer in the 200-km-long reach between the vicinity
of Omaha and St. Joseph that has been highlighted as “stable-aggrading” (Jacobson et al., 2009, Figure 8). Some of the sediment would be expected to become deposited behind the increasing number of bank structures that are being installed for shallow water habitat (see Chapter 4). The engineering activities that maintain the navigation channel are likely to ensure the mobility of this relatively fine sediment supply from the reservoir.
The enhanced washload would be flushed downstream efficiently in the leveed and simplified channel of the lower Missouri River. A small fraction of it would be stored in the floodplain in diffuse and channelized overbank flows, but even if the entire 6 MT/yr that might bypass Gavins Point dam were to reach St. Louis, it would constitute only a roughly 10 percent increase in the total sediment flux into the Mississippi from the Missouri. This amount is considerably smaller than the 34 MT/yr expected to be returned to the river by Corps projects for shallow water habitat, although at this point the habitat construction projects are slated to be conducted over a 15-year period.
Gavins Point Dam has been considered by some interest groups for removal to restore the downstream sediment supply. Although dam removal has been used to promote river restoration on some U.S. rivers, and more than 700 dams have already been removed, they have all been small (less than 30 feet high). The largest U.S. dams slated for demolition are Glines Canyon and Elwha dams on the Elwha River in Washington, which have a combined reservoir storage capacity of 48,000 acre-feet. By comparison, the storage capacity of Lewis and Clark Lake (impounded by Gavins Point Dam) is 492,000 acre-feet. There are no precedents for the removal of such a large structure, and if it were to be removed, augmentation of the sediment supply would again be only about 6 MT/yr (Coker et al., 2009), although the coarser sandy deposits at the upper end of the Niobrara River delta would eventually be washed downstream under this strategy. Only removal of all or most of the dams on the mainstem and tributaries would restore the sediment supply to the Missouri River at Yankton approximately to preregulation quantities. Only restoration of the coarsest bedload fraction of the river’s load would supply sediment that could remain for long on the bed of the degraded reach upstream of the Platte River; this generalization could be refined by totaling the volumes stored in successively further upstream dams. Restoration of even approximately natural sediment loading would require the unlikely (most energetically demanding) remobilization of the coarse fraction of the sediment input to each reservoir, which is deposited in deltas and fans at the upstream ends
of the reservoirs and in the delta and lowermost reaches of tributaries such as the Niobrara River.
Increasing Sediment Deliveries from Tributaries
Before the 1950s, tributaries supplied more than 175 MT/yr of sediment to the lower Missouri River, and even today when they are extensively impounded, they supply almost all of the 55 MT/yr that passes Hermann, Missouri (see discussion in Chapter 2). Sediment contributions vary between the diverse reaches within the major tributary basins because of the variety of their geology, soils, topography, and land use. Small tributaries on the left bank (east or north side) that drain loess areas contribute mostly fine grained sediment that passes through the system without settling on the channel bed. The Missouri’s right-bank tributaries contribute more sand. The Niobrara River contributes a relatively large sand load to Lewis and Clark Lake because its watershed is primarily the Sand Hills region of Nebraska. Substantially increased contributions of sediment from tributaries to the Missouri River downstream from Gavins Point Dam are unlikely under present sediment management because these rivers have large storage reservoirs of their own. Eighteen reservoirs control flow from 85 percent of the Kansas River basin (Perry, 1994), and two large impoundments (Harry S. Truman Reservoir and Lake of the Ozarks) control most of the flow of the Osage River.
With regard to prospects for bypassing sediment around dams on the Missouri’s right-bank tributaries, some sediment is already flushed around the Guernsey and Spencer dams on the North Platte and Niobrara rivers. Increased sediment bypassing of dams might aggravate flooding levels in the lower reaches of some tributaries, depending whether the bypassed sediment were washload, suspendible bed material, or bedload. The fact that these tributaries already supply more sediment to the lower Missouri River than could be supplied at steady state from Lewis and Clark Lake suggests that this source may be worth assessing in the future if additional options for reintroducing sediment to the Missouri are pursued.
MISSOURI RIVER SEDIMENT MANAGEMENT AND LOUISIANA WETLAND BUILDING
Since closure of the Missouri River mainstem dams and the construction of bank stabilization projects under the BSNP, Louisiana’s coastal wetlands have experienced substantial erosion and losses. Louisiana has lost 1,900 square miles of coastal wetlands since the 1930s (Barras et al., 2003). Between 1990 and 2000, wetland loss was approximately 24 square miles per year and the projected loss over the next 50 years (and accounting
for current restoration actions) is estimated to be approximately 500 square miles (Barras et al., 2003). These losses are of concern for several reasons, and the Corps of Engineers and state of Louisiana have conducted many studies and initiatives aimed at coastal protection and restoration. Given the historically important role of the Missouri River in delivering sediments to Louisiana, there is a strong perception that reductions in these sediment deliveries have been an important factor in wetlands losses. Some observers have suggested it might be possible to engineer an increase in the supply of Missouri River sediment to the threatened wetlands of the delta (Barry, 2008; Schleifstein, 2010). Others (e.g., Allison and Meselhe, 2010; Kim et al., 2009) have considered how sediment from the Mississippi River might be diverted out of the channel to reconstruct at least part of the coastal wetlands, even if there is no prospect of restoring their original extent with the available sediment supply (Blum and Roberts, 2009).
In addition to reduced sediment deliveries from the Missouri and Mississippi rivers, wetlands losses have been affected by a complex combination of other factors (see also NRC, 2009a):
sea level rise;
natural consolidation of soils;
reduced sediment delivery from the Mississippi River because of stabilization of the river banks;
construction of flood control structures along the mainstem Mississippi River in Louisiana that prevent flooding of wetland by sediment-laden waters and conveyance of river sediments to the delta front and over the edge of the continental shelf;
wetland edge erosion by storms that are likely exacerbated by the larger open water fetch in ever-enlarging distributary bays (e.g., Atchafalaya and Barataria);
navigation and pipeline canals cut through the wetlands by the oil and gas industry; and
offshore disposal of dredged materials by the Corps of Engineers.
Missouri River sediment clearly was important in contributing to maintenance of the elevation of Louisiana coastal wetlands. Given the many factors that have affected losses of these wetlands, however, the relative importance of reduced sediment deliveries to Louisiana remains difficult to quantify (Turner, 1997).
It is inconceivable that increases in the supply of Missouri River sediment from any strategy described in this chapter could reestablish a near-natural rate and volume of sediment delivered to the Mississippi River delta. As noted earlier in this chapter, full establishment of the Corps’ shal-
low water habitat program, and disposal of all of the excavated sediment into the channel and its complete transfer to the delta (unlikely because some of the suspendible sand is likely to be incorporated into the floodplain of the Mississippi River) would increase the sediment supply past Hermann, Missouri, by roughly 60 percent—or an additional 23 percent of sediment supply to the head of the delta—for at least 15 years (Jacobson et al., 2009).
If the SWH program was terminated, and then sediment was to be flushed past Gavins Point Dam, the sediment load past Hermann, Missouri, would increase by only 10 percent—roughly 4 percent of the load delivered to the head of the Mississippi delta. These values are maxima because the sand supply would be diminished by being incorporated into bars and the floodplain, and a portion of the washload could be lost to the vegetated floodplain.
Approximately 3.6 MT/yr of sand are dredged from the bed of the middle Mississippi River between the Missouri and Ohio river confluences, and the Mississippi River main channel in that stretch is narrow and meandering, with scattered chutes and backwater channels (Pinter et al., 2004). Most of this sand was dredged from flow divides, tributary mouths, and thalweg crossings. The fate of the dredged material is difficult to predict if it was returned within the channel margins, but it is likely to become involved in the floodplain during some part of its downstream transit. However, the dredged reach does not seem to offer much opportunity for the long-term storage of washload.
The lowermost 155 miles of the Mississippi channel have been held in place by dikes, with only limited accretion and erosion occurring along banklines inside the artificial levees, and ephemeral muds are temporarily deposited from suspension in water depths less than 60 feet along bank areas during low water (Nittrouer et al., 2008). Downstream of Baton Rouge, Louisiana (River Mile 230), the Mississippi River channel flows on local cohesive sedimentary formations with no long-term accumulation of sediment on its bed. Transient sediment on the bed is in the fine-to-medium sand range. Thus, although loss to overbank deposition would likely involve a large fraction of the washload in a natural river (e.g., Dunne et al., 1998), the levees along the Mississippi River increase the likelihood that most of the flow and its washload will stay in the channel.
Any significant increase in the quantities of sediment released from storage in the Missouri River mainstem reservoirs upstream of Lewis and Clark Lake would require remobilization of the silts and clays stored far upriver behind Fort Randall, Big Bend, Oahe, and Garrison dams. These reservoirs—Lake Francis Case, Lake Sharpe, Lake Oahe, and Lake Sakakawea—were recipients of fine-grained sediments that were washed abundantly from the soils developed on the Mesozoic shales of North and South Dakota. Because of their small grain size, these materials, once suspended,
would be mobilized as washload. Their greater distances upriver, however, and the necessity of their having to be bypassed through more than one reservoir and around the dams, would require extensive, expensive efforts in both planning and implementation.
The Corps of Engineers is implementing Emergent Sandbar Habitat and Shallow Water Habitat projects along the Missouri River consistent with the 2000/03 Biological Opinion Reasonable and Prudent Alternative. These projects aim to restore a portion of some features of the preregulation Missouri River to help protect endangered bird and fish species. As described in Chapter 2, prominent features of the pre-regulation mainstem Missouri River were a high sediment load traveling in suspension and along its bed, and highly turbid conditions.
These Emergent Sandbar Habitat and Shallow Water Habitat projects are reintroducing some sediment into the Missouri River, and are gradually reintroducing channel mobility and hydraulic connections between the main channel and its floodplain that support new habitat formation. This chapter discussed several other alternatives that might be employed to re-introduce additional sediments into the Missouri River. This chapter described and commented on sediment management alternatives independent of one another, and did not consider how different combinations of these alternatives might affect the sediment regime in the river or beyond the mouth of the river. Implementing combinations of these alternatives would require current Missouri River planning efforts (MRERP and MRAPS; see Chapter 3) to formulate and evaluate combinations of the actions discussed in this chapter, as well as other actions, at different scales and locations.
Primary alternatives that might be employed to reintroduce additional sediment into the Missouri River are: removing bank stabilization and control structures; limiting commercial dredging; bypassing sediment around mainstem dams; dam removal; and increasing sediment from tributaries.
Implementation of any of these alternatives would be constrained by financial, technical, and other factors. A major constraint on any alternative is the degree to which current economic activities, transportation infrastructure, and public safety depend on the existing system of dams and river bank control structures. It is not likely that major reconfiguration of the river channel, or removal of a large dam, would be desirable or acceptable to a large majority of Missouri River valley residents in the near future.
Bypassing of large amounts of sediment around Gavins Point Dam may be technically feasible. This option, however, would be expensive and have little potential to significantly reestablish preregulation supplies of sediment that were delivered to Louisiana. Substantially increased contributions of
sediment from tributaries to the Missouri River downstream from Gavins Point Dam are unlikely under present sediment management rules because these rivers have their own large storage reservoirs.
There has been a renewed interest in the prospects for increasing the amounts of sediment transported downstream by the Missouri River and delivered to Louisiana. However, there is little potential in the near future for any strategy described in this chapter to reestablish volumes of downstream sediment delivery that approach preregulation sediment volumes delivered to Louisiana. The Corps of Engineers Missouri River habitat construction projects could release enough sediment to increase the supply to the Mississippi delta by 10-20 percent for at least the next 15 years (depending on the trapping efficiency of the Mississippi floodplain). Remobilization of sediment in Lewis and Clark Lake would at best increase the supply of wetland constructing sediment to the Mississippi delta by only a few percent. Other prospects for mobilizing sediment in the Missouri and its tributaries are not only difficult to conceive of for the near future, but they are more likely to have local effects on bar building and local channel mobility and complexity than to contribute significantly to wetland construction in the Mississippi delta.
The amounts of sediment likely to be available for transport from the Missouri River to the Mississippi River delta are smaller than the quantities that made the journey before the construction of mainstem dams and implementation of the major bank-stabilization structures.