<br />LARRy W. HEsSE ET AI... 337
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<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).
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