8094

• UCREFRP Catalog Number: 8094
• Author: Dryer, M. P. and A. J. Sandvol.
• Title: Recovery Plan for the Pallid Sturgeon ‎(
• USFW Year: 1993.
• Copyright Material: NO

there was a clear relationship between poor year class development in most.
native species studied and the artificial hydrograph.
Before impoundment behind Missouri River reservoirs .(1926 to 1952), peak
discharges generally-occurred in April, and then-again with a larger peak in
June (Pflieger and Grace 1987)._ Today,, dam operations reduce flows from April
to July for flood control, and increase flows from July to April for
navigation, water supply, and hydropower: In addition to such seasonal .shifts
in the flow patterns, main stem dams operating for daily hydropower needs-can
cause daily fluctuations in water levels in tailwater areas by as much as 2 to
3 m (6.6 to 9.8 ft)`. This fluctuation can disrupt the macroinvertebrate
community and°larval fish rearing-areas for many miles downstream of the. dam
by alternately flooding°and dewatering habitats.
Modde and Schmulbach (1973) observed that factors affecting shovelnose
sturgeon prey availability within the unchanneli-zed Missouri River include
temperature, seasonal recruitment, and changes in density influenced by the .
timing and discharge rates from Gavins Point Dam. They hypothesized that the
reduction in numbers of shovelnose sturgeon may be due to reduced availability
of prey species caused. by high discharges from Gavins Point Dam.
Before .the Missouri River was channelized and impounded, it annually eroded
3.1 hectares/km of its floodplain (U.S. Army Corps of Engineers 1981). -Most
of this erosion has stopped due to channelization and impoundment. Erosion.
was a natural funct-ion of the river system,-and through erosion, inorganic
sediments, organic matter,. and large woody debris were introduced into the
river. This material import was essential to the habitat dynamics and
nutrient cycling of the river system.- Such sediment and nutrient discharge
are the raw materials for habitat development in the Missouri and Mississippi.
River system. Construction of dams eliminated 80 percent of this material.
Fremling (1989) reports that the sediment load of the middle Mississippi River
has declined 66 percent from pre-1935 levels, mainly due to sediment-
entrapment in Missouri River impoundments. This lack of sediment delivery-
upset the natural channel equilibrium and was replaced by a variety of
nonequilibrium processes such as hydraulic sorting and bed paving, which
eventually will eliminate all sediment movement. This has already occurred to
some extent and has resulted in reduced bed roughness-and, therefore, reduced
substrate diversity. This has. reduced the reproductive success of substrate
spawners, such as sauger (Stizostedion canadense), sturgeon, and paddlefish
(Hesse and Mestl 1993b).
The turbidity caused by suspended sediment also provided the pallid sturgeon
and othew native fish, adapted to living in a nearly sightless world,..with-
cover:while moving from one snag or undercut bank to another. Today,-water
clarity has increased dramatically,-and this essential cover is gone.. Under
such conditions,, predation by sight-feedsng predators, such as northern pike-
(Esox-Lucius), walleye (Stizostedion vitreum), and.smallmouth bass
(raicropterus dolomieui); can be expected to significantly impact native.
species not equipped by errolution with good eyesight.
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