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Nicasio Reservoir was created by the construction of Seeger Dam, an earthen filled dam on Nicasio Creek in 1960. The reservoir's current storage capacity is 22,340 acre-feet. The proposed Nicasio Spillway Modification project would install a 280-foot-long, 4.4-foot-high inflatable rubber gate spanning the width of the existing spillway crest of Seeger Dam to increase the capacity at Nicasio Reservoir by approximately 3,700 acre-feet. The proposed project calls for the District to raise the inflatable gate during the wet season to catch additional rainfall.
Learn how this project fits into Marin Water's broader Water Supply Roadmap – a long-term strategy for building water resilience.
Environmental Review Process
Pursuant to the California Environmental Quality Act, Marin Water initiated environmental review of the proposed Nicasio Spillway Modification Project in May of 2025.
Update (8.21.25) – Work to develop the Nicasio Spillway Modification Project has stopped - Significantly higher costs than originally anticipated The District will retain all work completed to date for future reference, but the project has been withdrawn from CEQA review and removed from active planning -- this includes any survey work that was going to inform the environmental review and planning for this proposed project. Though not required, to ensure clarity, the District has contacted the State Clearinghouse to withdraw the Nicasio Spillway Modification Notice of Preparation and sent notice of that action to all interested parties on the project email list. Information on this page has been modified to reflect this update. |
Benefits
Improved drought resilience - the additional water supply yielded from the proposed project would help increase the District's water supply resiliency during future droughts.
Background
Marin Water supplies water to more than 191,000 people in central and southern Marin. About 75% of the District's water supply originates from rainfall on the Mount Tamalpais Watershed and in the hills of west Marin. That water flows into the District’s seven reservoirs, which hold nearly 80,000 acre-feet of water. Supplies from Marin Water's local watersheds are generally sufficient in most years and of high quality.
Marin Water supplements its local rainfall supplies with water imported from the Russian River system in Sonoma County and with recycled water. Together these supplemental supplies account annually for about 25 percent of Marin Water’s supply. Through its award-winning Water Efficiency programs, Marin Water also works to reduce demand.
Historically, Marin Water has successfully met water supply demands during periods of extreme drought with a combination of conservation and increased Sonoma Water supplies. However, Marin Water’s most recent drought risk assessment indicates that even with implementation of its established water shortage contingency plan, Marin Water would still experience a supply shortfall during periods of extreme drought. In addition, recent studies indicate that severe droughts are projected to become much more frequent as a result of changing climate conditions.
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