East Fork Russian River is a long tributary of the Russian River in Mendocino County, California artificially connected to the Eel River via an interbasin diversion at the Potter Valley Project hydroelectric facility. It forms in the north of Potter Valley, flows south through this valley, then southwest through a mountain pass to Lake Mendocino, an artificial reservoir that empties into Russian River. At one time Clear Lake to the east drained through Cold Creek then along the lower part of East Fork Russian River through Coyote Valley to the Russian River proper. A few hundred years ago a massive landslide blocked this channel, and Clear Lake found a new outlet to the Sacramento River.
Cold Creek flows year round, while the upper part of East Fork Russian River used to dry up in the summer leaving isolated pools along its course. This changed when the Potter Valley Project was completed in 1908. The project involved construction of two reservoirs on Eel River to the north of the Russian River basin, and a tunnel to the head of Potter Valley that carried water from Eel River to a hydroelectric station that discharged into East Fork Russian River. The Eel River water was used to irrigate Potter Valley, and supplied water to Ukiah below the convergence with Russian River. In 1958 Coyote Dam was built on East Fork Russian River just above its mouth on Russian River, flooding Coyote Valley to form Lake Mendocino. The lake provides additional water storage as well as flood control.
The continuous flow from the Potter Valley Project changed the fish ecosystem in the Russian River by eliminating the warm, still pools that had been used for breeding by native fish. In the 1950s the state introduced poison to the river to eliminate "useless" fish species in favor of game species. The Coyote Dam prevented steelhead trout from returning upstream, so today the East Fork Russian River is dominated by rainbow trout. The section of the river that flows through the mountains between Potter Valley and Lake Mendocino includes stretches of white water that are challenging for kayakers and rafters.
Watershed
The East Fork Russian River is a long tributary of the Russian River in Mendocino County. It drains the northeastern part of the Russian River basin. The region has a Mediterranean climate. The fork's watershed covers . The mouth elevation is about above sea level. East Fork Russian River forms in the north end of Potter Valley and flows south through a canyon to Coyote Valley, now filled by Lake Mendocino.
Potter Valley is an alluvial valley, a depression created by movement of parallel faults that has filled with gravel, boulders and cobble eroded from the surrounding mountains. The basement rock is Franciscan Complex, a jumbled mixture of different types of rock that is very prone to erosion and landslides. Coyote Valley is a southerly trending valley about wide by long that is underlain primarily by metamorphic rocks from the Franciscan formation. Most of the recreation areas located within the Lake Mendocino boundary have 6 to 12 inches of silt, or sandy silt, overlying the gravelly phase, Older Alluvium.
Coyote Valley is flanked by rolling hills that rise about the valley floor to the west of Lake Mendocino and steeper Franciscan bedrock hills to the east. Elevations above mean sea level in the surrounding region range from about in the valleys near Ukiah to about on top of Cow Mountain to the east. The hills to the east of Lake Mendocino are very rugged and continue for many miles. To the west and northwest, the hills are more rounded with benches that were once planted with vineyards.
In a period of very low water in September 1905 the East Fork Russian River had a discharge of . A discharge measurement station is located upstream from the Russian River and downstream from the Coyote Dam, which impounds Lake Mendocino. At this point the maximum discharge of the East Fork Russian River before regulation by Lake Mendocino was , recorded on 21 December 1955. Since then the maximum discharge was , recorded on 24 January 1970.