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

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¯ VOLUME II, CHAPTER 30
is being considered near Casa Diablo, Mono Craters, and survival. About thirty such dams were built on the Eldorado
Bridgeport Valley in Mono County. National Forest (1980).
Dams are constructed in a great range of sizes for various
purposes. Dams of a few meters’ height are found throughout
the Sierra Nevada to improve hydraulic conditions for
D A M S A N D D I V E R S I O N S
tunnel intakes diverting water for municipal supply or ~rri-
gation or toward powerhouses. Such dams or weirs are not
intended to have any effect on the seasonal pattern of stream
Impounding and diverting of streams are the principal im- flow. Dozens of small dams for small-scale hydroelectric propacts
on the hydrologic system of the Sierra Nevada. While duction were proposed throughout the Sierra Nevada under
other resource management activities cause environmental the favorable climate created by the Public Utility Regulaalterations
that, in turn, may affect stream flow, water manage- tory Policy Act of 1978 (California Energy Commission 1981).
ment activities avoid the intermediate steps and in- Many of these projects were ill conceived and were based on
tentionally and directly alter the hydrologic regime. The thor- unrealistically high projections of future energy prices. Only
oughness of the hydraulic engineering in the Sierra Nevada a small proportion of those proposed were ever built. Several
that has been developed over a century and a half is probably existing and proposed hydroelectric projects are being reconunderestimated
by most water users in California. However, sidered by their owners or proponents because of currently
one simple fact stands out: no rivers reach the valley floor low prices for electricity. The larger diversion dams can store
unaltered. Only three Sierra Nevada rivers greater than 65 stream flow accumulated over a few days. Dams intended to
km (40 mi) long flow freely without a major dam or diver- redistribute water over time have storage capacities equivasion:
Clavey, Middle Fork Cosumnes, and South Fork Merced lent to the stream flow of at least several weeks. A few of the
Rivers (figures 30.2-30.4). Selected segments of the North Fork megaprojects can hold more water than is produced in an
American, Middle Fork Feather, Kern, Kings, Merced, and average year. These massive structures account for most of
Tuolumne Rivers receive some protection from additional the storage in an entire river basin. For example, the New
dams under the National Wild and Scenic River System Melones Reservoir has 84% of the total storage capacity in
(Palmer 1993). Few streams get very far from their source the Stanislaus River Basin, while the next largest forty dams
before meeting some kind of structure. In the Mono Lake and in the basin represent only the remaining 16% (Kondolf and
Owens River Basins, about 730 km (460 mi) out of 850 km Matthews 1993). The dam at Lake Tahoe controls only 1.8 m
(530 mi) of streams are affected by water diversions (Inyo (6 ft) of storage, but the vast area of the lake makes its storage
National Forest 1987). In California’s Mediterranean climate, volume the ninth largest in the Sierra Nevada. The big dams
water is most available in winter and spring. Dams are built in the Sierra Nevada cause many of the same problems as
to reduce the peak flows of winter, provide irrigation water other large dams in the western United States (e.g., Hagan
during the growing season, provide domestic and industrial and Roberts 1973). These dams prevent the further migration
water on a semiconstant basis, allow optimum hydroelectric of anadromous fish and completely change the water and
generation, and secure some interannual storage for protec- sediment regimes downstream. The combined effects of all
tion against drought. With so many uses of water, attempts the large dams on rivers tributary to the Sacramento River
to manage it are found throughout the Sierra Nevada. have significantly modified the annual hydrograph of the largest
river in the state (Shelton 1994).
Structures
The other critical structures in water management are the
conduits and canals for transferring water between rivers or
The total number of water management structures in the Si- to powerhouses or users. The vast network of artificial waerra
Nevada is unknown but must be in the thousands. The terways redistributes water over short and long distances. A
storage capacity of all dams in the range is about 28 billion water molecule can take a very circuitous journey from the
m 3 (23 million AF), which ~s about the average annual stream mountainside to the valley through several p~eces of the
flow produced in the range. The dozen largest reservoirs (each plumbing system. Many of the old ditches and canals origiwith
capacity greater than 500 million m 3 [400,000 AF]) ac- nally constructed during the mining era and that still supply
count for about three-fourths of the rangewide storage ca- water for hydroelectric generation, municipal use, or irrigapacity.
The smallest dams in the Sierra Nevada are those built tion have become a secondary channel system. They both
for minor domestic water supply on small creeks and may collect water from and discharge water to soils and slopes. In
impound only a few cubic meters of water. Somewhat larger a 160 km (100 mi) long canal network in El Dorado County,
dams have augmented natural lakes and were often built for about half of the initial water plus any gains en route are lost
fisheries management purposes. Most of these dams were con- to seepage (Soil Conservation Service 1984). Water-supply
structed before World War It, but a few continued to be built agencies have sought to increase the efficiency of their anup
to the 1960s. Their main purpose was to store water for tique delivery systems by reducing seepage from the old
releases in late summer to maintain some stream flow for fish ditches. Replacement of the open ditches with pipes avoids
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