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<br />there was a clear relationship between poor year class development in most <br />native species studied and the artific al hydrograph. <br /> <br />Before impoundment behind Missouri River reservoirs (1926 to 1952), peak <br />discharges generally occurred in April, and then again with a larger peak in <br />June (Pflieger and Grace 1987). Today, dam operations reduce flows from April <br />to July for flood control, and increase flows from July to April for <br />navigation, water supply, and hydropow\ir. In addition to such seasonal shifts <br />in the flow patterns, main stem dams operating for daily hydropower needs can <br />cause daily fluctuations in water leve)s in tai1water areas by as much as 2 to <br />3 m (6.6 to 9.8 ft). This fluctuation: can disrupt the macroinvertebrate <br />community and larval fish rearing areas for many miles downstream of the dam <br />by alternatelY flooding and dewatering habitats. <br /> <br />Modde and Schmulbach (1973) observed that factors affecting shovel nose <br />sturgeon prey availability within the .unchannel ized Missouri River include <br />temperature, seasonal recruitment,andchanges in density influenced by the <br />timing and discharge rates from Gavins Point Dam. They hypothesized that the <br />reduction in numbers of. shovel nose st~rgeon may be due to reduced availability <br />of prey species caused. by high discha~ges from Gavins Point Dam. <br /> <br />Before the Missouri River was channelized and impounded, it annually eroded <br />3.1 hectares/kin of its floodplain (U.S. Army Corps of Engineers 1981). Most <br />of this erosion has stopped due to channelization and impoundment. Erosion <br />was a natural function of the river system, and through erosion, inorganic <br />sediments, organic matter, and large woody debris were introduced into the <br />river. This material import was essential to the habitat dynamics and <br />nutrient cycling of the river system.: Such sediment and nutrient discharge <br />are the raw materials for habitat development in the Missouri and Mississippi <br />River system. Construction of dams eliminated 80 percent of this material. <br /> <br />Frem1ing (1989) reports that the sediment load of the middle Mississippi River <br />has declined 66 percent from pre-1935 levels, mainly due to sediment <br />entrapment in Missouri River impoundments. This lack of sediment delivery <br />upset the natural channel equilibrium and was replaced by a variety of <br />nonequilibrium processes such as hyd~aulic sorting and bed paving, which <br />eventually will eliminate all sedlme~t movement. This has already occurred to <br />some extent and has resulted in reduqed bed roughness and, thereforB, reduced <br />~ubstrate diversity. .This has reduced the reproductive success. of substrate <br />,.spawners, such as sauger (Stfzostedfon canadense), sturgeon, and paddlefish <br />.. (Hesse and Mestl 1993b). .. <br /> <br />The turbidity caused by suspended sediment also provided the pallid sturgeon <br />and other native fish, adapted to li~ing in a nearly sightless world, with <br />cover while moving from one snag or ~ndercut bank to another. Today, water <br />clarity has increased dramatically, and this essential cover is gone. Under <br />such conditions, predation by sight-feeding predators, such as northern pike <br />(Esox 7ucius), walleye (Stizostedfonivftreum), and smallmouth bass <br />(mfcropterus do7omfeuf), can be expected to significantly impact native <br />species not equipped by evolution with good eyesight. <br /> <br />12 <br />