D--048966 - CALFED Bay-Delta Program - State of California

890
VOLUME II, CHAPTER 30
cases, low flows are augmented, and the streams run at unnaturally
high (and often constant) levels throughout the year.
Sediment Storage and Transport
Below some hydroelectric powerhouses, discharges related Reservoirs dramatically change the sediment transport regime
to power demands can fluctuate wildly over a few hours, of a river. Virtually all bedload and most suspended load is
Afterbays allow regulation of the water released to the river, deposited when a river enters the still water of a reservoir.
Reservoir releases that partially resemble a natural The coarser fraction of the sediments forms a delta at the uphydrograph
probably have the least adverse impact on down- per end. Deposits tend to be progressively finer toward the
stream ecological processes. The minimum (and relatively dam. When the water level is lowered, the streams cut through
constant) flow requirements on many water projects may the deposits and relocate materials closer to the dam. Under
serve to keep fish alive, but they are quite different from the some conditions, this channel incision can progress upstream
flow regime that the aquatic community evolved with. Greater (Galay 1983). With the extensive water development in most
consideration of downstream ecological needs could be in- river basins of the Sierra Nevada, changes in sediment delivcorporated
in the operations of many reservoirs without in- ery should be considered throughout the basin. Each dam in
curring major costs. The opportunities for alterations in release ~’~ the network affects the channel below it. With the presence of
exploredSChedulingonanda project-by-projecttheir potential benefitSbasis, and costs need to be
~many dams upstream, contributing areas for sediment are
\often much smaller than contributing areas for water (in the
Changes in the flow regime also impact water quality. When ~.,,,absence of exports out of the basin).
stream flow is diminished substantially, there is less volume Most of the geomorphic adjustments to dams occur downavailable
to dilute contaminants entering downstream. In stream. These channel changes occur in response to shifts in
1994, the U.S. Supreme Court decided that states had the au- sediment delivery and flow regimes, especially peak flows.
thority under the Clean Water Act to regulate reservoir re- Whatever water is released has significantly less sediment
leases in the context of managing water quality. In some cases, than when it entered the reservoir. Unless releases are minireservoirs
can improve riverine quality by allowing contami- mal, the sediment-free discharge has the capacity to entrain
nants adsorbed on particles to settle out of the water column, and transport particles from the bed and banks of the down-
However, this same process may be converting some reser- stream channel. Progressive lowering or degradation of the
voir beds into storage deposits of heavy metals (Slotten et al. riverbed may occur after dam completion. Typical conse-
1994, 1995). quences of degradation include lowering of ground-water
levels and consequent loss of riparian vegetation, reduction
Water Temperature in overbank flooding and deposition of sediments and nutrients,
bank erosion and loss of land, exposure of bridge foun-
Temperatures of streams below dams are affected by the vol- dations, and abandonment of diversion intakes (Galay 1983).
ume and temperature of reservoir releases. If little water is The severity of channel incision depends on the size distrireleased
from a dam in summer, streams can become unnatu- bution of particles in the bed, characteristics of the channel,
rally hot because the radiant energy of sunlight on the chan- how the reservoir is operated and the sequence of flood events
nel is absorbed by a smaller volume of water than under following construction (Williams and Wolman 1984). Downnatural
flow conditions. Dams may be designed to release cutting seems to be greatest in rivers with fine-grained bed
water from different depths in the lake. Reservoirs become materials and where flood peaks are not greatly reduced by
thermally stratified like most natural lakes. In summer, stored the dam. However, larger dams usually reduce flood peaks
water tends to be warmer near the surface, so releases from substantially and thereby limit the rate of degradation
upper levels will result in higher temperatures than releases (Milhous 1982). Where channel incision occurs below dams,
near the base. the finer particles are removed, and the larger cobbles and
boulders are left behind. As the bed becomes coarser or "ar-
Evaporation mored," it is more resistant to erosion and interferes with
salmonid spawning. Also, downcutting decreases the chan-
The reservoirs behind the dams also export water to the atmosphere.
Lakes lose water by evaporation roughly in pro-
~ nel gradient slightly, and degradation becomes somewhat self-
limiting. The bed of the Yuba River below Englebright Dam
portion to their surface area. Creation of large expanses of has become armored with large cobbles and boulders but is
open water by damming a river can significantly increase the still susceptible to incision during the largest floods (Kondolf
opportunities for water losses from a watershed. Up to a meter and Matthews 1993). Conversely, flood control is so effective
of annual evaporation can be expected from reservoirs in most "- below both Pardee and Camanche Dams that channel degraof
the Sierra Nevada (Harding 1935; Longacre and Blaney dation has not occurred and the gravels are immobile
1962; Myrup et al. 1979). (BioSystems Analysis 1990, cited by Kondolf and Matthews
1993). Unfortunately, we lack any information about the condition
of channels before placer mining and dam construc-
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