Seeger Spillway Modernization Project

Marin Water is planning a long-term modernization project for Seeger Dam’s spillway at Nicasio Reservoir. The project will upgrade portions of the spillway to meet modern spillway standards, ensuring the facility continues to operate safely and reliably for decades to come. The project is being developed in close coordination with the California Division of Safety of Dams (DSOD), the state agency responsible for regulating dams throughout California. 

Following the 2017 Oroville Spillway failure, DSOD directed dam owners throughout California to perform additional evaluations of spillways with similar construction as Oroville Spillway. Seeger Dam was one of three Marin Water dams identified for detailed spillway assessment. The multiphase evaluation included record reviews, field inspections, subsurface investigations, and structural analyses completed over several years. Based on those findings, DSOD directed Marin Water to develop a modernization plan for the Seeger Dam spillway. 

After reviewing the results of the spillway condition assessment, DSOD concluded that portions of the Seeger Dam spillway, including the underdrain system and several other design components, do not meet current spillway design standards. Based on those findings, DSOD changed Seeger Dam’s condition rating from “Satisfactory” to “Fair” and directed Marin Water to develop a modernization plan. 

Yes. Seeger Dam continues to operate under the oversight of the California Division of Safety of Dams (DSOD) and is inspected annually as required by the State of California. DSOD’s current “Fair” condition rating means no existing dam safety deficiencies are recognized under normal operating conditions. The planned modernization project will upgrade the spillway to current spillway design standards, helping ensure it continues to perform safely and reliably for decades to come. 

DSOD assigns condition ratings for regulated dams throughout California. According to DSOD, a Fair rating means: “No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency.” A “Fair” condition rating does not mean the current condition of the dam is unsafe, that dam failure is expected, or that emergency repairs are needed. Rather, it indicates that modernization is appropriate to improve long-term performance and resilience under all conditions, including rare or extreme. 

The findings were identified through a phased engineering evaluation initiated by Marin Water at DSOD’s direction following statewide guidance issued after the 2017 Oroville Spillway failure. The assessment occurred over several years as follows: 

• Phase I (2017-2021): A high-level evaluation of the Seeger Dam, Peters Dam and Soulajule Dam spillways identified routine non-structural maintenance items, including crack repairs, joint sealing and drain cleaning. Those non-structural spillway repair activities were completed in 2021 in accordance with DSOD guidance. 
• Phase II (2022-2025): At DSOD’s direction, a more detailed subsurface investigation evaluated the Seeger Dam, Peter’s Dam, and Soulajule spillway’s structural condition using ground penetrating radar, concrete coring and video inspections of the underdrain system. The completed engineering reports were submitted to DSOD in April 2025. 
• DSOD Review & Guidance (2025-2026): After reviewing the Phase II report for Seeger Dam, DSOD updated Seeger Dam’s condition rating from “Satisfactory” to “Fair” and directed Marin Water to develop a modernization plan for the spillway. Marin Water met with DSOD in June 2026 to discuss the findings and begin the modernization planning process. DSOD is still reviewing the Phase II report for Soulajule Dam and Peters Dam. 

The current modernization project is the result of this multi-year engineering and regulatory process, with each phase building on the findings of the previous phase under DSOD’s oversight. 

• Learn more about the initial phases of the Dam spillway assessment.

Seeger Dam continues to operate under the oversight of the California Division of Safety of Dams (DSOD). The updated condition rating does not change the dam’s normal operations. Marin Water will continue working closely with DSOD throughout the modernization project to ensure the facility is operated in accordance with all applicable state requirements. Consistent with emergency preparedness coordination, Marin Water has notified the Marin County Office of Emergency Management of the updated condition rating. Like all regulated dams in California, Seeger Dam has a state-required Emergency Action Plan and inundation maps that support emergency preparedness and coordination with local response agencies. 

The California Division of Safety of Dams (DSOD) are the experts on dams and they regulate more than 1,200 dams statewide and is responsible for reviewing dam safety evaluations, conducting annual inspections, and overseeing dam improvement projects. Marin Water is developing this project in close coordination with DSOD, which provides technical oversight throughout planning, design and construction. 

Specific work will be determined during the engineering design process. Current planning focuses on modernizing the spillway, including upgrades to the underdrain system and other components identified during the condition assessment to meet current spillway design standards. 

Consistent with similar dam spillway modernization projects throughout California, the project is currently estimated to take approximately 8 – 10 years. This includes engineering studies, alternatives analysis, environmental review, permitting, detailed design, regulatory review by DSOD and construction.  

Marin Water will provide updates as the modernization project progresses through planning, environmental review, design and construction. Project updates will be shared through Board meetings, the Marin Water website and other Marin Water public communication channels. 

Like all regulated dams in California, Seeger Dam has a state-required Emergency Action Plan (EAP) that identifies and supports emergency preparedness and coordination between Marin Water, the California Division of Safety of Dams (DSOD), California Office of Emergency Services (CalOES), Marin Office of Emergency Management, first responders and local water agencies for mutual aid. More detailed information on Dam Safety Planning, including EAPs is available on the California Governor’s Office of Emergency Services website. EAPs and inundation maps are required for all regulated dams throughout California and are maintained as part of standard emergency preparedness planning. Their existence does not indicate that Seeger Dam or any other Marin Water dam is unsafe or that dam failure is expected, but rather provide Marin Water, State and local authorities guidance on how to manage events. 

Inundation maps show the areas that could potentially be affected in the unlikely event of a dam failure. They are used by emergency management agencies to support emergency planning, preparedness and response. Like all regulated dams in California, Seeger Dam has state-required inundation maps and an Emergency Action Plan that are maintained in coordination with the California Division of Safety of Dams (DSOD), California Office of Emergency Services (CalOES) and local emergency management agencies. The existence of these maps does not indicate that a dam is unsafe. They are a requirement for emergency preparedness. Inundation maps are available on the California Department of Water Resources Division of Safety of Dams website. 

Following the Oroville Spillway failure, DSOD directed spillway assessments at three Marin Water dams with similar spillway construction: Peters Dam, Seeger Dam, and Soulajule Dam. Each dam is unique and evaluated independently by DSOD. Seeger Dam is the only Marin Water dam whose condition rating was changed as a result of this assessment.