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<br /> <br />16 <br /> <br />OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN <br /> <br />Water Surface <br />Profile <br /> <br />Shoreline <br />Location <br /> <br />Unregulated river <br /> <br />Moderate <br /> <br />Low <br /> <br />High <br /> <br />- -- <br /> <br />-'- <br /> <br />-', <br /> <br />.~~m point control <br />'-, <br />--. <br />--- <br /> <br />nDDI <br /> <br />Midreach control point <br /> <br />nne <br /> <br />--. <br /> <br />"- <br /> <br />Flow <br /> <br />Low flow <br />Moderate flow <br />High tlow <br /> <br />away by currents and waves. Sediment resuspension also <br />decreases the depth of lighl penetration in Ihe water. further <br />reducing aquatic plant production (Roseboom and others. <br />1992), Species occurrence and habitat associations are <br />reported for two LTRMP study reaches in the upper Ilood- <br />plain reach and Illinois River (Langrehr. 1992; Peitzmeier- <br />Romano and others. 1992; Shay and Gent. 1992), They are <br />also available for other UMR reaches (LTRMP. unpub, <br />data). Waler-Ievel regulation may have reduced weIland <br />plant abundance by limiting the exposure of mudIlats dur- <br />ing low discharge periods, <br /> <br />INVERTEBRATES <br /> <br />Invenebrates are relatively abundant in the upper <br />Iloodplain reach (LTRMP, unpub. data). Although there <br />have been periods when certain species (fingernail clams) <br />have declined. Ihey appear to have recovered (Eckblad and <br />Lehlinen, 1991; LTRMP. unpub, data), The mechanisms for <br />these population fluctuations are not yet understood. I <br />suspect that the abundance of epiphytic invertebrates <br />increased dramatically following impoundment because of <br />Ihe great expansion of plant beds, I also suspect that Ihey <br />and their fish food value declined with the decline of the <br /> <br />Flow <br /> <br />figure 2-7. Water-I~vel response under three levels of control. <br />The pooled reaches of the Mississippi River exhibit hydrologic <br />zonation based on floodplain gradient and proximity to the down- <br />stream (impounding) dam. Shorelines in unregulated rivers are <br />determined by floodplain topography and discharge. Dam point <br />control maintains stable water levels at the downstream dam. <br />Upper pool reaches occupy the upstream one.third to one-half of a <br />pool; river stage responds mostly to discharge~ependent water <br />releases Ihrough the upstream dam. Midpool reaches exhibit some <br />flooding during moderate flows, but flooding is attenuated down- <br />stream toward lower pool reach, where water levels are maintained <br />relatively stable by Ihe dam. The same low.flow pool shape is <br />maintained in pools regulated at midpool control points, but a <br />major ecological pcnurbation occurs during moderate flows. <br />Water releases through the downstream dam are increased to <br />reduce flooding within the pool reach, and drawdowns occur in the <br />lower pool reach. Orawdowns reduce flooding in midpool reaches. <br />but the lower pool is drained, and shallow backwaters are exposed <br />at a time of year when river habitats should be flooded. Some dams <br />operate with a combination of dam and midpool control points. At <br />some level of high discharge, all darns go to "open-river" condi- <br />tions. where the dam has liule effect on river stage. There is some <br />leeway between open river and severe flooding because of flood <br />easements and structural control devices. Most dams operate under <br />open.river conditions during each year because dams are only used <br />to raise water-surface elevations during low and moderate dis- <br />charge periods. The three methods of control indicate some level <br />of flexibility in water-level management Implementation of dam <br />point control on dams currently regulated al midpool would <br />increase the floodable area within a pool reach. Fish and wildlife <br />management could benefit through careful coordination with navi- <br />gation system needs. <br /> <br />wetland plants. They have been replaced by organisms more <br />adapted to open-water habitats with silty substrates (chi- <br />ronomids and aquatic wonns). Annual production of aquatic <br />invertebrates may be reduced because of the lack of a Ilood <br />pulse to inundate terrestrial habitats and transport energy 10 <br />the midpool and lower pool reaches. <br /> <br />FISHES <br /> <br />Lentic fish populations increased dramatically immedi- <br />ately following impoundment. The combination of <br />expanded backwater habitat and high plant and invertebrale <br />production supported large populations of popular game and <br />commercial fish species (Sparks. 1992), As with plants and <br />invertebrates, however, fish populations began to decline <br />with the aging of the navigation pools, Movement of some <br />migratory species (blue sucker. lake sturgeon. American eel. <br />skipjack herring. paddlefish. and pallid sturgeon) was <br />impacted when the dams were constructed. Nonmigralory <br />species have been affected by the loss of habitat (PIlieger. <br />1975; Dillard and others. 1986), Today Ihe upper Iloodplain <br />reach offers some of the best game fishing opportunities in <br />the UMR, bUI they do not equal reports of the opportunities <br />found immediately after the dams became operalional. Fish <br />