THEIR DOCUMENTS. THEIR WORDS.

How Much NERF Water Can Lake Mendocino Actually Store?

The Eel-Russian Project Authority (ERPA) says future Eel River diversions depend on rainfall and Lake Mendocino’s storage capacity. What does that mean for water available during the dry season?

ERPA proposes a new facility to divert Eel River water into the Russian River watershed after PG&E decommissions the Potter Valley Project. On its own FAQ page, ERPA says anticipated diversion volumes depend on rainfall levels and Lake Mendocino storage capacity. It describes up to 30,000 acre-feet per year as anticipated and says the facility could reliably divert up to 50,000 acre-feet under typical wet-season conditions.

Those figures deserve a closer look. Water that can be diverted during a wet period is not necessarily water that can be stored for summer use. The amount retained depends on when the water arrives, how much room Lake Mendocino has at that time, reservoir operating rules, and the water already flowing into the lake.

ERPA’s Water Diversion Agreement rules change by season: they set minimum Eel River flows before diversion can begin and limit the share of flow that can be diverted. The rules also set a 300-cubic-foot-per-second facility limit. They describe potential diversions under specified conditions; they do not, by themselves, show how much water Lake Mendocino would retain through a sequence of dry years.

LPA believes the public should be able to see that full calculation. For each water year, how much water is eligible for diversion, how much reaches Lake Mendocino, how much can be retained, and how much remains available when Russian River communities need it most?

These are questions about the performance of a proposed water system. They deserve answers grounded in daily data, reservoir operating rules, and clearly stated assumptions.

Read the documents yourself: ERPA Fact Sheet-Summer 2025, ERPA FAQ and the Water Diversion Agreement, Appendix 3.