How the Russian River works: dams, minimum flows and floods

By The Downspout · Updated Oct 12, 2026 · 7 min read

Two dams store winter rain and release it in summer. Sonoma Water draws the water from wells beside the river for over 600,000 people. The state sets how much must stay in the river.

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Check the river today: Russian River level and forecast at Guerneville (National Weather Service).

Schematic drawn from Sonoma Water, FERC and State Water Board sources listed below.

The pieces

Lake Mendocino. Coyote Valley Dam was finished in 1959. The lake holds up to 118,000 acre-feet. 70,000 of that is for water supply.
Lake Sonoma. Warm Springs Dam sits on Dry Creek. The lake holds up to 381,000 acre-feet, with 245,000 for water supply. Sonoma Water's history page says it began storing water in 1983. Its water supply page says 1984.
Potter Valley Project. A PG&E hydropower project. Since 1906 it has sent Eel River water into the East Fork of the Russian River.
Collector wells. Six wells beside the river, near Wohler Bridge and Mirabel Park, draw water from the sand and gravel under and beside the river.
Pipes. Over 88 miles of pipeline carry it to cities and water districts. The first, the Santa Rosa Aqueduct, started deliveries on May 24, 1959.

How often the river floods at Guerneville

Our calculation from National Weather Service historic crests for the Guerneville gauge. Method: highest listed crest in each water year (October to September), 1940 to 2025, counted if 40 feet or more.

The weather service calls 32 feet flood stage at Guerneville and 40 feet a major flood.
The river hit major flood stage in 14 of the 86 water years from 1940 to 2025. That is about 1 year in 6.
It passed flood stage in at least 37 of those years. The crest list is not complete, so the true count may be higher.
The record is 49.5 feet on February 18, 1986, and 102,000 cubic feet per second.
That is about 800 times the lower river's Normal-year minimum of 125 cubic feet per second.
The most recent crest above flood stage in these records: 38.79 feet on February 5, 2025.

The dams hold back some winter water, but they do not stop big floods. Four of the six highest crests on record came after Coyote Valley Dam was finished in 1959.

How much Eel River water still comes in

Our calculation from the State Water Board order of May 20, 2026. Method: (150,000 minus 40,000) divided by 150,000. Both figures are Sonoma Water's, as given in the order. Recent transfers average less than 40,000, so the real drop is larger.

When the state set river flow rules in 1986, about 150,000 acre-feet a year came over from the Eel. That is about 49 billion gallons.
Transfers now average less than 40,000 acre-feet a year, Sonoma Water told the State Water Board. The order names infrastructure failures and seismic risk at the project.
On July 25, 2025, PG&E asked federal regulators (FERC) to let it give up the project and remove Scott Dam and Cape Horn Dam.
The plan includes a new facility at the Cape Horn Dam site to keep some Eel water coming. The Eel-Russian Project Authority would run it.
FERC took scoping comments through September 18, 2026. Its page shows no decision yet.

With less Eel water coming in, the temporary 2026 state order judges whether a year is Normal or Dry by how much water is in Lake Mendocino. Before, it used inflow to Lake Pillsbury on the Eel River.

How much water has to stay in the river

Since 1986 the two dams have been run to meet minimum river flows set by the State Water Board in Decision 1610. The minimum depends on whether the year is Normal, Dry or Critical.

Stretch of river	Normal	Dry	Critical
Upper river, Lake Mendocino to Dry Creek	75 to 185	75	25
Lower river, Dry Creek to the ocean	125	85	35

Minimum flow in cubic feet per second. Source: State Water Board order, May 20, 2026, which lists the Decision 1610 terms. In Normal years the upper river minimum ranges from 75 to 185, depending on the month and how much is stored in Lake Pillsbury and Lake Mendocino.

A temporary state order of May 20, 2026 cut the Normal-year minimums to 125 on the upper river and 70 on the lower river. It lasts 180 days, to about November 16, 2026.
Sonoma Water has filed a petition for the same cuts starting May 1, 2027. See our news post on the 2027 petition.

Who runs it

Sonoma Water runs the water supply: the wells and the pipes. It began on October 1, 1949 as the Sonoma County Flood Control and Water Conservation District. Floods from 1935 to 1945, with $6.1 million in damage, were part of the reason.
PG&E runs the Potter Valley Project.
The State Water Board sets the minimum flow rules.

What the river means for salmon and steelhead

In the late 1990s coho salmon were listed as endangered here, and Chinook salmon and steelhead trout as threatened, under the federal Endangered Species Act. That shapes how the river is run.

More water is not always better for young fish. The National Marine Fisheries Service's 2008 Biological Opinion found the 1986 summer minimums "too high for optimal juvenile salmonid habitat."
Fast summer currents leave young salmon fewer calm places to feed and grow.
Big summer releases also use up the cold water in Lake Mendocino. Fish need that cold water later, in late summer and fall.
The same opinion found that high summer flows harm habitat in the estuary where the river meets the ocean.
These findings are a main reason Sonoma Water asks for lower summer minimums. The 2025 opinion found its plan "not likely to jeopardize" the listed fish.

Sonoma Water started a Fisheries Enhancement Program in 1995.

What it means for you

If you get water from a city or district, it very likely starts here. See where your tap water comes from.
Water you save stays in the lakes for dry years and for fish. Sonoma Water says use across the Sonoma-Marin Saving Water Partnership was 7% below 2020 from January to June 2026. Start with rebates or laundry to landscape.
If you live near the river, watch the Guerneville forecast in winter storms. Home Ready Sonoma has a winter storm checklist for getting a house ready.
A strong El Niño is likely this winter. See what strong El Niños have meant for river floods.

Next: find who supplies water to your address.

Acre-foot: about 325,851 gallons, the water that covers an acre one foot deep.