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14 OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN U.S, Army Corps of Engineers Districts SI. Paul District II Rock Island District 800 750 4 Qj 700 > .!!l 650 .. Q) '" 600 Q) > 550 0 Minneapolis .0 .. 500 SI.Paul Q) If 450 400 350 17 18 19 II SI. Louis District 15 16 27 Low water of /9-IOotChannel Low water before .... 9-footcllarmel .... Riverbed Approximately 670 miles SI. Louis Cross section diagram of 9-foot channel, upper Mississippi River Figure 2-5. Cross section of the upper Mississippi River navi~ gation system illustrating changes to water-surface elevations throughout the "pooled" portion of the UMR. Low-discharge water-surface elevations were increased between St. Louis. Mis- souri, and Minneapolis. Minnesota. to an elevation necessary to maintain a 9-fool-deep navigation channel along the system's 670- been heavily affected by sediments dropped from suspen- sion in the slow flowing lower pool (fig, 2-9), Backwaters contiguous with the river or subject to inundation by flood- waters suffer from the same effect. The result has been sig- nificant losses of water depth (Adams and Delisio. 1990) and creation of flocculent silt substrates that are easily resuspended by boat- and wind-generated waves (Adams and Delisio, 1990; Bhowmik and others. 1990), The basin was affected by urban development. logging. and agricul- ture, which increased sediment delivery from the basin. WETLAND HABITATS Submersed aquatic and wetland herbaceous plants initially thrived in the newly expanded aquatic habitats, The adverse effects of sedimentation became apparent about 20 years after the dams were built. when macrophyte beds began to decline in the upper floodplain reach, Silty sedi- ments, built up through the period of impoundment. provide poor habitat for rooted aquatic plants that can be scoured mile length. High-discharge water-surface elevations (stage) are unaffected by the navigation dams, and they all go to "open river" (see text) at some point in most years. Because high discharge stages are unaffected. the river retains much of its unregulated fonn and function (source: U.S. Anny Corps of Engineers, 1992). Figure 2-4;. Predam (1891) and postdam (1989) land-cover/- land.use maps of the Pool 8 reach. These maps document changes in river.tloodplain habitats caused by navigation dams. Although somewhat developed near LaCrosse, Wisconsin, the river in the late 1800's showed great habitat diversity and patch- iness. The marsh class represents a conglomerate of wetland plants that were probably susceptible to inundation on a fre- quent (annual) basis. Regulation for maintenance of the 9.[00t- deep navigation channel fanned the broad, open expanse in the lower half of the navigation pool. At midpool. permanent inun- dation is less evident except in complex braided channels fOlmed by the raised water table surfacing in the varying flood- plain topography. The upper pool reach is relatively unaffected by the downstream dam. Habitat diversity is reduced in the large open-water area created upstream of the dam. Flow regu- lation has allowed suspended sediments to settle out and accu. mulate in slackwater habitats. Waves erode islands and resuspend fine sediments, which block light needed by aquatic plants and algae. Midpool and upper pool reaches offer wave- protected habitats that support higher habitat diversity through the maintenance of aquatic and wetland plant communiti~s.