Description of Proposed Project

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. Modifying Nicasio Reservoir’s spillway is one of the near-term storage enlargement projects identified in the District's Water Supply Roadmap. 

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. 

Benefits

If approved to move forward, the additional water supply yielded fromthe project would help increase the District's water supply resiliency during future droughts. 

Environmental Review Process 

Pursuant to the California Environmental Quality Act, Marin Water is initiating environmental review of the proposed Nicasio Spillway Modification Project. 

Public Comment Period for the Scope of the Environmental Review 

The review begins with a comment period for agencies and the public to comment on the scope of the environmental review, including the project description, environmental issues to be analyzed and potential alternatives. The issues are described in a Notice of Preparation. 

The public comment period is open April 30, 2025, and until 5 p.m. Aug. 4, 2025. Comments must be submitted before the deadline and may be provided in one of the following ways: 

• By email to: 
  Elysha Irish at eirish@marinwater.org 
  Subject: Nicasio Spillway Modification Project public comment 

OR 

• By U.S. postal mail to: 
  Marin Water 
  Attention: Elysha Irish 
  220 Nellen Ave. 
  Corte Madera, CA 94925 

Scoping Meetings 
As part of the environmental review process, Marin Water held three public scoping meetings for interested parties to learn more about the project and provide public comments. The information presented was the same for all three meetings.

Presentation for EIR Scoping Meetings 

• Scoping Meeting 1 – Thurs, May 22 – Video Recording
• Scoping Meeting 2 – Tues, May 27 – Video Recording
• Scoping Meeting 3 – Tues, July 15 – Video Recording
   

Ongoing Surveys 

Various surveys are needed to inform the environmental review and planning for this proposed project. These surveys include environmental surveys, topographic boundary surveys, and surveys of Halleck and Nicasio Creeks. 

The table below provides additional information about the surveys including what the survey entails, its current status, the purpose, and the location of the properties that Marin Water proposes to survey. The surveys will provide critical insights to support Marin Water’s efforts to gain a more comprehensive understanding of the surrounding area as we investigate the potential effects of the proposed project. 

For each survey, property owners will be contacted on an individual basis for property access as more detailed timing is known as it relates to each identified parcel.

Survey type: Environmental

Survey type: Boundary

Survey type: Creeks

Outreach and Engagement Presentations