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Nicasio Spillway Modification Project

Aerial image shows a dam and spillway connected to a reservoir.

Project Description 

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. This 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. 

 

Update (May 15, 2025): Public Comment period extended, additional public meeting date added for Nicasio Spillway Modification Project's Environmental Review Scoping

Marin Water is extending the public comment period and adding another public scoping meeting for the proposed Nicasio Spillway Modification Project. The extension will allow more time to obtain suggestions and information from agencies and the public on the content and scope of issues to consider in developing the proposed project’s Environmental Impact Report, which Marin Water is initiating work on, pursuant to the California Environmental Quality Act. The public comment period was originally scheduled to close on June 4, 2025 and will now close on August 4, 2025. An additional virtual scoping meeting is now also being added to the schedule for July 15; this is in addition to the two scoping meetings that were already previously scheduled. This webpage has been updated to reflect the changes in the public comment period schedule. 

 

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

Comment Period

The public comment period will be open April 30, 2025 until 5:00 p.m. on August 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 is holding two public scoping meetings. These meetings are an opportunity for interested parties to learn more about the project and provide public comments. The information presented will be the same for both meetings.

 


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

The project is a relatively cost-effective way to achieve additional water storage capacity in a shorter timeframe as compared to other, longer-term water supply alternatives within the District’s Water Supply Roadmap. The additional water supply yielded would help increase the District's water supply resiliency during future droughts. 
 

Estimated Schedule and Status

Marin Water is currently developing the designs of the proposed spillway raise, conducting the environmental review of the project, and coordinating with regulatory agencies and potentially affected landowners. 

Public comment on the scope of the environmental review and Notice of Preparation will run from April 30, 2025 and until 5:00 p.m., August 4, 2025. 

Planning & Design:  Summer 2024 - Spring 2026
Construction:  2027
 

Funding Sources 

Capital Maintenance Fund 
Other Funding Sources 
Total
$15,000,000
- 
$15,000,000
ProjectNicasio Spillway Modification Project 
ProgramGeneral Improvement | A1A05 Dams and Reservoirs
Project NumberD25007
ContactMatthew Steiner | msteiner@marinwater.org