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 turbines in 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 159000 acre feet, although this figure varies significantly with both the amount of precipitation in the Eel River basin and the demand on the Russian River.

In 2019, PG&E chose not to relicense the hydroelectric project with the Federal Energy Regulatory Commission, as it had become unprofitable to operate. Federal regulations require that PG&E submit a final license surrender application by 2025, which would enable it to begin decommissioning the project. The dams and hydroelectric facilities are to be removed, while various options are being considered for maintaining water diversions to the Russian River.

History
Construction on the project began in 1900, when 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 ft-diameter tunnel under the drainage divide to Potter Valley, at the headwaters of the East Fork Russian River. The water dropped 450 ft to a powerhouse before being released to the East Fork Russian River. On April 1, 1908, the first deliveries were made and power production began with a capacity of 4000 kilowatts (KW). In 1910, the generation capacity was boosted to 7000 KW and in 1912 second penstock was built to increase the flow capacity of the tunnel. The powerhouse was upgraded to 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 mi 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.

The Federal Energy Regulatory Commission relicensing of the project on January 28, 2004, placed limits on the amount of water that can be diverted. In combination with drought conditions, diversions between 2004 and 2009 averaged 90000 acre feet, or 57% of the historical average. Since then, late summer water has been released from Cape Horn Dam at rates roughly mimicking or exceeding natural flows in an attempt to mitigate the impacts to fisheries.

Operations
The project derives water from a drainage basin of 289 mi2 above Scott Dam and approximately 50 mi2 between Scott Dam and Cape Horn Dam, where water is diverted to the Russian River. The vast majority of the water arrives as winter rain between December and April, with a smaller, less reliable amount furnished by snowmelt and groundwater through June. Scott Dam, which forms Lake Pillsbury, has a total storage capacity of 74993 acre feet. Project regulations require that the gates at Scott Dam be opened between October 16 and April 1, for safety reasons during the winter months. Winter storms fill the reservoir, which provides only very limited flood control, because the average annual runoff of 400000 acre feet is over five times the project storage capacity. It is not uncommon for the dams to spill eight or nine times during a single winter season. After the wet season passes, Lake Pillsbury is drawn down beginning April 1. Typical summer drawdowns leave the reservoir at or above 20000 acre feet, or 27 percent capacity. Water is released to Cape Horn Dam, which diverts the majority, while releasing a small flow to the Eel River designed to mimic natural summer flows. This is typically around 20 cuft/s, but can decrease significantly during dry years.

Beneficiaries
In 1924, the Potter Valley Irrigation District (PVID) was formed to provide irrigation water to the farmers along the East Branch Russian River. The district currently serves 390 farmers with rights to 22670 acre feet of project water per year, for the irrigation of 4905 acre within a district boundary of 6900 acre. Because there is very little natural runoff in Potter Valley and the local geology is non-conducive to groundwater storage, the PVID is the only constituent that depends solely on Eel River water.

Project water also serves farmers and municipalities downstream along the Russian River, in Mendocino County. The total water use per year is about 17000 to 23000 acre feet. Even further downstream, water users in Sonoma County use between 50000 and 80000 acre feet per year. These users depend both on Potter Valley Project water and natural flows in the Russian River basin managed by the RRBP. In addition to agricultural, domestic and industrial uses, project water helps to maintain a minimum dry season flow of 150 cuft/s in the Russian River, serving for recreational, aesthetic and fishery enhancement purposes. Project water is estimated to provide at least part of the water supply for nearly 500,000 people living in Sonoma and Mendocino Counties, mainly in the North Bay area, for cities such as Santa Rosa.

Environmental impact
The Potter Valley Project has had a significant impact on summer low streamflows in the Eel River basin. Although the project taps water from only the upper 10% of the Eel River system, this headwaters region provides most of the summer flow in the lower Eel, especially during critically dry years, when only 5 cuft/s are allowed into the Eel River past the Cape Horn diversion point. As a result, summer-run salmon and steelhead in the Eel River are negatively affected during dry years. In addition, while Cape Horn Dam has a fish ladder, the larger Scott Dam blocks fish migration to about 100 mi of habitat in the Eel River headwaters.