Translate
Capturing Winter Water to Build a More Resilient Supply
As climate change brings more extreme swings between wet and dry years, Marin Water is proposing a new infrastructure project to better capture and store water during the rainy season. The Atmospheric River Capture (ARC) Project would build a 13-mile pipeline connecting the existing North Marin Aqueduct – where Russian River water from Sonoma Water Agency is conveyed – to Nicasio Reservoir, making it possible to store surplus winter water for later use during dry months and drought years.
By storing water that would otherwise flow to the ocean, the project increases Marin's available supply.
Learn how ARC fits into Marin Water's broader Water Supply Roadmap – a long-term strategy for building water resilience.
Why it matters
Even in dry years, atmospheric river storms can bring large bursts of rainfall to Northern California. These events often cause the Russian River to temporarily swell with far more water than is needed to meet environmental flow requirements – yet much of that water currently flows out to the ocean, unused.
This project is possible because the Russian River draws from a much larger watershed – close to 1500 square miles – compared to Marin Water's 33-square-mile local catchment. That broader area produces reliable winter flows even during droughts, according to historical records. Importantly, this water is not drawn from existing storage, but instead comes from seasonal rainfall and runoff.
Marin's reservoirs don't always fill during the winter, especially in dry years. During times like these, the ARC Project would allow capture of surplus winter water during key storm windows and store it in Nicasio Reservoir – helping boost local supply in years when our system would otherwise fall short.
Building Resilience Locally and for the Region
Marin Water came dangerously close to exhausting its local water supply during the 2021-2022 drought, prompting a water shortage emergency and some of the most severe water use restrictions in the District's history. That experience underscored the need for additional, reliable supply options – especially ones that can perform even in dry years.
The ARC Project is expected to be one of the most impactful projects identified in the District's Water Supply Roadmap. By capturing excess winter water from the Russian River and storing it locally, this project provides a critical buffer during droughts, supports longer-term planning and can strengthen the District's ability to weather an uncertain climate future.
The project could also support additional dry year water supply needs for North Marin Water District and creates an opportunity for other water utilities in the region to bank additional supplies.
How it Works
The ARC Project would construct a pipeline to move water from the existing North Marin Aqueduct into Nicasio Reservoir. By adding a dedicated connection and pumping system, Marin Water could capture excess winter flows and store them for use later in the year, including during droughts – something we cannot do today.
How it's Different from Our Current Russian River Supply
Marin Water currently imports about 20% of its annual supply from Sonoma Water. This water is imported from the Russian River and is delivered directly into Marin Water's distribution system for immediate use. There is no infrastructure to send that water into local reservoirs. The ARC Project adds that missing link – making it possible to store surplus winter water in Nicasio Reservoir for future use.
Project Details at a Glance
Source: Russian River winter flows (October through May)
Destination: Nicasio Reservoir in West Marin
Conveyance: Approximately 13 miles of new underground pipeline
Pumping Upgrades: Improvements to existing facilities like the Kastania Pump Station to support seasonal water transfers and construction of two new pumping facilities
Initial Yield: Up to 3,800 acre-feet per year, depending on rainfall, streamflow availability and reservoir conditions
Future Potential Yield: Up to 8,100 acre-feet per year with expanded conveyance and pumping capacity
Benefits
Improved Drought Resilience – Captures surplus winter water, such as during atmospheric river events, and stores it in local reservoirs for use during extended dry periods
Regional Collaboration – Strengthens regional water resiliency by connecting to existing infrastructure shared with neighboring agencies, creating opportunities to support mutual aid and coordinated planning of access to shared water sources during future droughts.
Environmental Review
As required under the California Environmental Quality Act (CEQA), Marin Water will conduct a full Environmental Impact Report (EIR) for the proposed project. This process will evaluate potential impacts on wildlife, waterways, air quality, traffic and other factors. It will also include opportunities for public input.
Estimated Timeline
The ARC Project is currently in the early preliminary design stages to further evaluate the project. The estimated timeline below outlines key milestones:
| Phase | Estimated Timeframe |
| Preliminary Design & Review | 2025 - 2026 |
| Environmental Review (CEQA) | 2025 - 2027 |
| Final Design & Permitting | 2026 - 2027 |
| Construction | 2027 - 2029 |
| Anticipated Operation | Mid to late 2029 |
*Timeline subject to change based on environmental review, easements and property acquisitions and permitting.
Stay informed
Learn how ARC fits into Marin Water's broader Water Supply Roadmap
Review Board/Committee meeting recordings and documents from past project updates