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Missouri River Planning: Recognizing and Incorporating Sediment Management
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 SEDIMENTREINTRODUCTION 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