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<br />LARRy W. HEsSE ET AI... 337 <br /> <br />1954, dominant discharge occurred only twice in <br />33 years. The result has been the stabilization of <br />the channel's morphological configuration. Dy- <br />namic change was stopped nearly 40 years ago. <br />Native fish and wildlife used the historical channel <br />components (sandbars, chutes, pools, backups, <br />dunes, islands) as essential habitat. <br />We recommend a return to a semblance of the <br />natural hydrograph. Initially this change could be <br />based on a daily percentage of the mean annual <br />discharge during a precontrol period. This ap- <br />proach would allow recovery of the seasonality of <br />flows while providing control over the magnitude; <br />however, dominant discharge must be recovered, <br />and development of a floodplain corridor is essen- <br />tial for this process to be restored in part. <br />Through fine tuning of the navigation channel, <br />as much as two-thirds of the flow of the river <br />during July-October could be stored in the main- <br />stem reservoirs to be used to emulate the spring <br />flood pulse in riverine reaches. We believe this can <br />be done with only minimAl effect on full service <br />navigation (Hesse 1992), and the draft results of <br />Master Water Control Manual modelling by the <br />U.S. Army Corps of Engineers suggests that <br />power generation losses will be minimAl (U.S. <br />Army CorpS of Engineers 1992). <br /> <br />We recommend that the U.S. Army Corps of <br />Engineers investigate sediment bypass systems <br />for the Missouri River and its tributaries. Sedi- <br />ment bypass is feasible (Singh and Durgunoglu <br />1991), and additional benefits such as increased <br />water storage in hydropower reservoirs; elimina- <br />tion of delta formations in the upper end of reser- <br />voirs, which can cause lowland flooding; and re- <br />duced degradation,' which will reduce navigation <br />channel maintenance costs, damage to water in- <br />takes and bridge abutments, and head cutting in <br />tributary streams. <br />Options for study may include operating lake <br />discharge as run-of-the-river for a year, sluice <br />gates (below grade at the dam), sluice bypass <br />channel, and sluice pipeline (on or in the lake bed); <br />for a short-term solution, land adjacent to the <br />river channel just downstream from the dam can <br />be acquired and pushed into the river channel. <br /> <br />Altered Water Temperature <br /> <br />Loss of Sediment Transport <br /> <br />The largest dams on the mainstem of the Mis- <br />souri River release water from. depths of 42 m (Fort <br />Randall Dam) to 59 m (Oahe Dam; U.S. Army <br />Corps of Engineers 1985). Cold bottom strata have <br />significantly altered downstream riverine water <br />temperature. <br />Water temperature was 210 C at river km 1,112 <br />(on the channelized reach downstream from Sioux <br />City, Iowa) and 230 C at river km. 801 on the same <br />day in May 1987, and 260 C at river km 1,112 and <br />280 C at river km. 801 in June 1987. This reach <br />runs nearly straight north and south, and the <br />effect on warming because of latitude would be <br />greatest in this reach. Under the same circum- <br />stances, we measured water temperature at river <br />km 1,393 (31'km downstream from Fort Randall <br />Dam) to be 100 C in May 1987 and 160 C at river <br />km 1,178 (216 km. nearly due east). Water tem- <br />perature was 170 C at river km 1,393 and 260 C at <br />river Ian 1,178 on the same day in June 1987. <br />Thermal modification of this magnitude can affect <br />aquatic insects by altering emergence cues, egg <br />hatching, diapause breaking, and maturation <br />(petts 1984). Native fish, such as sauger, stur- <br />geon, blue sucker, and others, spawn in response <br />to water temperature, photoperiod, and run-off <br />cues. Today these cues send mixed signals. We <br />recommend that selective withdrawal be incorpo- <br />rated into existing dam-reservoir design. Water <br />could be discharged from near the surface, or <br />bottom water could be mixed with surface water <br /> <br />Dams on the mainstem and tributaries have <br />short-stopped the movement of sediment from up- <br />stream. The precontrol river was in a' state of <br />equilibrium; net sediment entering a reach re- <br />placed an equal amount leaving. Sand, silt, and <br />organic matter were the raw materials for habitat <br />development and aquatic nutrition. Precontrol av- . <br />erage annual suspended sediment loading was 149 <br />million metric tons at yankton, South Dakota, and <br />grain size averaged 20% sand, 40% silt, and 40% <br />clay. By 1954, annual suspended sediment loading <br />dropped 81% to 30 million metric tons. The sand <br />fraction more than doubled, while silt and clay <br />were halved (Slizeski et al. 1982). In addition to <br />eliminating much of the material for habitat devel- <br />opment, areas downstream from. dams and the <br />lower ends of tributary streams have developed <br />severe channel bed degradation. Degradation has <br />contributed to the loss of off-channel habitat and <br />has furthered the severance of the floodplain- <br />channel connection (U.S. Army Corps of Engineers <br />1991). <br /> <br /> <br />'~ <br /> <br />1 <br /> <br />l~ <br />