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Factors Affecting the Species Range Wide <br />Habitat Loss and Degradation - Destruction and alteration of big -river ecologic functions and habitat <br />once provided by the Missouri and Mississippi Rivers is believed to be the primary cause of declines in <br />reproduction, growth, and survival of pallid sturgeon (USFWS 1993). The physical and chemical <br />elements of channel morphology, flow regime, water temperature, sediment transport, turbidity and <br />nutrient inputs once functioned within the big -river ecosystem to provide habitat for pallid sturgeon and <br />other native species. Today on the main stem of the Missouri River, approximately 36 percent of <br />riverine habitat within the pallid sturgeon's range has been transformed from river to lake by <br />construction of six massive earthen dams by the Corps between 1926 and 1952 (USFWS 1993). <br />Another 40 percent of the river downstream of dams has been channelized. The remaining 24 percent <br />of river habitat has been altered by changes in water temperature and flow caused by dam operations. <br />The channelized reach of the Missouri River downstream of Ponca, Nebraska, once a diverse <br />assembledge of braided channels, sandbars, and backwaters, is now confined within a narrow channel <br />of rather uniform width and swift current. Morris et al. (1968) found that channelization of the Missouri <br />River reduced the surface area by approximately 67 percent. Funk and Robinson (1974) calculated <br />that the length of the Missouri River between Rulo, NE, and its mouth ( -500 RM) (3 10 km) had been <br />reduced by 8 percent, and the water surface area had been reduced by 50 percent following <br />channelization. <br />Missouri River aquatic habitat between and downstream of main stem dams has been altered by <br />reductions in sediment and organic matter transport/deposition, flow modification, hypolimnetic <br />releases, and narrowing of the river through channel degradation. Those activities have adversely <br />impacted the natural river dynamics by reducing the diversity of bottom contours and substrate, slowing <br />accumulation of organic matter, reducing overbank flooding, changing seasonal flow patterns, severing <br />flows to backwater areas, and reducing turbidity and water temperature (Hesse 1987). The Missouri <br />River dams also are believed to have adversely affected pallid sturgeon by blocking migration routes <br />and fragmenting habitats (USFWS 1993). <br />Levee construction on the lower Mississippi River from the Ohio River to near the Gulf of Mexico has <br />eliminated the river's major natural floodway and reduced the area of the floodplain connected to the <br />river by more than 90 percent (Fremling et al. 1989). Fremling et al. (1989) also report that levee <br />construction isolated many floodplain lakes and raised river banks. As a result of levee construction, <br />15 meander loops were severed between 1933 and 1942. <br />The pattern of flow velocity, volume, and timing of the pre - development rivers provided the essential <br />life requirements of native large -river fish like the pallid sturgeon and paddlefish. Hesse and Mestl <br />(1993b) found a significant relationship between the density of paddlefish larvae and two indices (timing <br />and volume) of discharge from Fort Randall Dam. They concluded that when dam operations caused <br />112 Status Range Wide -PS <br />