Potter Valley Project
The Potter Valley Project is an interbasin water transfer project in Northern California in the United States, delivering water from the Eel River basin to the headwaters of the Russian River. The project is owned and operated by Pacific Gas and Electric Company (PG&E). The main facilities are two dams on the Eel River, a diversion tunnel and hydroelectric plant. Average annual throughput is 159,000 acre·ft (196,000,000 m3), although this figure varies significantly with both the amount of precipitation in the Eel River basin and the demand on the Russian River.
Construction on the project began in 1900, when the natural flow of the Russian River was no longer enough to meet growing agricultural and urban demands. The Eel River Power and Irrigation Company (later the Snow Mountain Water and Power Company) constructed the Cape Horn Dam and a one-mile (1.6 km), 8-foot (2.4 m)-diameter tunnel under the drainage divide to Potter Valley, at the headwaters of the East Branch of the Russian River. The water dropped 450 feet (140 m) to a powerhouse before being released to the Russian River. On April 1, 1908, the first deliveries were made and power production began with a then-installed capacity of 4000 kilowatts (KW). In 1910, the generation capacity was boosted to 7000 KW and in 1912 second was built to increase the flow capacity of the tunnel. The powerhouse was finally upgraded to its present capacity of 9400 KW in 1917, after the addition of a fourth unit.
Initially, the project could only operate during the winter months, when there was enough water in the Eel River to divert without drying up the riverbed downstream. In 1920, Snow Mountain Water and Power began construction on a larger dam on the Eel River, 12 miles (19 km) upstream from Cape Horn. Scott Dam, which forms Lake Pillsbury, was completed in 1922. With its greater storage capacity, it provides water for the diversion during the summer months and also affords some flood control during winter storms. In 1930, ownership of the project was transferred to PG&E. In 1959, Coyote Valley Dam was built on the Russian River as part of the separate Russian River Basin Project (RRBP), forming Lake Mendocino, which provides additional storage of diverted Eel River waters. This reservoir serves a critical function during dry years as it is drawn down to compensate for reduced diversions from the Eel River system.
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