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

891
Hydrology and Water Resources
tion. Therefore, we are unable to make definitive statements of the sediments in a stable location has been a costly approach
about what constitutes natural channel conditions in most of to the problem. Ralston Afterbay on the Middle Fork Amerithe
Sierra Nevada, although channels were unlikely to have can River has had sediment removed on six occasions between
been as armored as many are currently,
Where streambeds are not armored with large materials
its completion in 1966 and 1986 (Georgetown Ranger District
1992). The average annual rate of filling of about 80 m3/km2 and are not actively degrading, fine sediments can interfere (0.2 AF/mi2) is not excessive compared with that of other
with fish spawning. Salmonids require gravels with sufficient basins, but the Ralston Afterbay has a capacity of only 3.4
pore space to allow interstitial flow to bring oxygen to eggs. million m3 (2,782 AF) with 530 km2 (205 mi2) of unregulated
Fine particles may be deposited between the gravels and limit contributing area above it (EA Engineering, Science, and Techthe
flow of water. Higher discharges are necessary on occa- nology 1990). Location of suitable sites for long-term storage
sion to cleanse the gravels. Control of high flows by dams~ of removed sediments within a short distance from theresereliminates
the opportunity to flush the fine sediments out of J voir has been difficult (Georgetown Ranger District 1992). The
the spawning gravels. Many studies have been conducted in ! small forebays on Southern California Edison’s Bishop Creek
the past decade to define how much water is needed for this ~ system have also required dredging of accumulated sediflushing
function, and many rules-of-thumb have been sug- ments. Estimates of the costs of dredging and transportation
gested. However, variability in fluvial processes among
streams illustrates the need to actually observe flows that
depend on access and distance to a disposal site and have
ranged from $26/m3 ($20/yd3) begin to entrain particles of a particular size rather than de-
(EAEngineering, Science, and
Technology 1990) to about $3,500/m3 ($2,700/yd3) (Kondolf
°
pend on generalized procedures to estimate flow releases and Matthews 1993).
necessary to remove fine sediments from spawning gravels Another option for removal of accumulated sediments ~
(Kondolf et al. 1987). sluicing. Opening sluice gates or an outlet tunnel allows wa- i
Limiting the size and frequency of floods below dams has ter levels to fall and sediment to be resuspended and flushed ~
also altered conditions for riparian vegetation. As total dis- out with thewater. This action creates a sudden pulse of sedi- \,
charge and scouring flows decrease, riparian vegetation is able ment downstream. Problems have arisen when sluicing has
to become established in the former active channel (Williams been conducted during summer months, at times when flows
and Wolman 1984). Roots stabilize the bank materials, and are inadequate to disperse the redeposited sediment. Sluicthe
plants slow overbank flows, which allows deposition of ing of Forbestown Reservoir on the South Fork Feather River
additional sediment. Gradually, the channel becomes nar- in 1986 left a thin layer of sand over the entire channel well
rower, and large trees occupy former parts of the channel. If downstream of the dam. Another example was Democrat Dam
allowed to become well established, mature riparian vegeta- on the Kern River in 1986. In the years following sluicing,
tion can resist significant flows. Confining the stream to a high flows did not occur, and sand remained within the channarrower
channel can increase hydraulic forces on the bed nel until scouring flows occurred in 1992 (Kondolf and
and lead to incision and loss of riparian vegetation. To some Matthews 1993). Accidental releases of sediment occurred ont
degree, dams mimic the effects of long-term droughts on the Middle Yuba River from Hour House Reservoir in 1986
vegetation-channel interactions (Mount 1995). Depending on and from Poe Dam on the North Fork of the Feather River in
characteristics of the channel and plants, establishment of ri- 1988. More than $1 million was spent excavating sand out of ~-~
parian vegetation can be enhanced by either higher or lower the channel below Hour House Dam, but a flood during the
summer flows than occurred before dam construction. En- early stages of the North Fork Feather cleanup conveniently
croachment of vegetation into river channels has been noted flushed all the excess sediments out of the channel (Ramey
below Tulloch, Don Pedro, La Grange, and McClure Reser- and Beck 1990; Kondolf and Matthews 1993).
voirs (Pelzman 1973). Augmentation of flows at the receiving When sediment is flushed out of reservoirs at low flows, it
end of trans-basin diversion has widened channels and has wilt be redeposited close to the dam; however, when it is inpushed
back riparian vegetation, as in tkie case of the upper troduced at higher flows, it will usually be carried down-
Owens River. stream and dispersed. Engineering approaches to letting
Although larger dams seem to have sufficient space to store sediments pass through dams during high flows are being
sediment for hundreds of years, at least at rates determined considered at several sites. The Pacific Gas and Electric Cornin
the 1940s, smaller structures can become overwhelmed with pany (1994) is designing pass-through systems to retrofit two
sediment in just a few years. Unusually large floods can corn- of its dams on the North Fork Feather River. Sediment is rappletely
fill smaller diversion works, as occurred at Log Cabin idly filling the reservoirs, complicating operation of the dams,
Dam on Oregon Creek and Hour House Dam on the Middle and accelerating turbine wear (Harrison 1992). Such pass-
Yuba in 1986 (Kondolf and Matthews 1993). Assuming that through systems could allow reservoir operations to interthe
dam is to remain in operation, the accumulated sediment fere less with natural sediment transport and could have
must be removed. How that removal is accomplished can have geomorphic benefits with regard to channel degradation bean
assortment of impacts. Dredging, trucking, and disposal low dams (Kondolf and Matthews 1993).
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