Connecting winter water to storage for a more resilient water supply
About the Project
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.
What’s New
Environmental Impact Report to be prepared for Atmospheric River Capture Project
Pursuant to the California Environmental Quality (CEQA) Act, Marin Water initiated environmental review for the Atmospheric River Capture (ARC) Project in March. This process evaluates potential impacts on wildlife, waterways, air quality, traffic and other factors. It also include opportunities for public input throughout the process.
As the CEQA Lead Agency, Marin Water will prepare the Project’s Environmental Impact Report (EIR) in compliance with CEQA. The draft EIR is currently expected to publish in summer or fall of 2026 and the next public comment period will open at that time.
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 1,500 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.
How it works: From storm to storage
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.
The Project would generally operate only when all of the following infrequent but critical conditions are present:
- Low storage levels across Marin Water’s reservoirs
- An extended dry period in Marin with little to no rain in the forecast
- Forecasted atmospheric river storms in Sonoma County’s Russian River watershed that elevate river flows
*Specific operating decisions would be based on a combination of these and other system-wide factors.
To understand more about how the project would operate, advance the slides below by clicking on the side arrows.
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.
What it builds: Connecting winter water to storage
The Project would include construction and operation of an approximately 13.2-mile-long, 36-inch diameter water transmission pipeline within Wood Hollow Drive, Redwood Boulevard, San Marin Drive, Novato Boulevard, Point Reyes–Petaluma Road, and Nicasio Valley Road. The pipeline, referred to as the Nicasio Aqueduct, would convey water from the existing North Marin Aqueduct to Nicasio Reservoir. In addition, the Project would include two pump stations and a dechlorination facility.
The Project would not increase the existing storage capacity of Nicasio Reservoir and would not modify the District’s existing agreement with Sonoma Water. The Project would deliver contract water from Sonoma Water for storage in Nicasio Reservoir during periods of high-flow events (i.e., atmospheric river events) for use by the District during dry conditions to improve water supply reliability.
Dry year water supply yield
The project could yield up to 3,800 acre-feet of water in a dry year, depending on rainfall, streamflow availability and reservoir conditions
Benefits
- Improve Drought Resilience – Captures surplus winter water, such as during atmospheric river events, and stores it in local reservoirs for use during extended dry periods
- Strengthen 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.
- Align with Community Priorities – Marin Water is coordinating with local agencies as the project develops to identify opportunities, where feasible, to align the project with local priorities and provide added community value.
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.
North Bay and broader regional water systems are interconnected, and agencies regularly work together during droughts, emergencies, and other periods of urgent need to help maintain reliable water supplies. By storing available winter water locally during wet periods, the project can help improve drought preparedness in Marin while supporting more flexible use of shared regional water supplies during future dry conditions.
Marin Water and neighboring agencies have a long history of working together to support regional water resilience during changing conditions, droughts and emergencies.
Drought Response (2021) – Residential Preparedness
During the most recent drought, Marin Water and North Marin Water District worked with the County of Marin and other partners to prepare for providing emergency potable water supplies for rural residents relying on wells that were at risk of running dry. These measures were ultimately not needed due to changing conditions.
Drought Response (2021) – Agricultural Use
Both water districts also worked with the County of Marin to support water needs in areas outside both districts’ service areas, including allowing West Marin ranchers to purchase and truck untreated reservoir water for agricultural use over several months during the drought.
Novato Landslide (2023)
A landslide in Novato raised concerns about access to local water supply. Marin Water coordinated with North Marin Water District and was prepared to support emergency interconnections if needed to help maintain service for North Marin Water District customers.
Future Opportunity – Stafford Lake Winter Water Storage
The ARC Project would include infrastructure connecting to Stafford Lake, creating a potential future opportunity for North Marin Water District to more efficiently capture and store available winter water during key storm events. By leveraging infrastructure used by both agencies, this could support additional drought resilience and more efficient use of available regional winter water sources.
Future Opportunity – Supporting North Bay Water Resilience
By allowing Marin Water to store water captured during key winter storm events for later use, the ARC Project could help reduce reliance on shared regional water supplies during droughts. This added flexibility may help support broader North Bay water resilience during periods when water supplies are limited.
Environmental Review Process
Pursuant to the California Environmental Quality Act, Marin Water initiated environmental review for the Atmospheric River Capture (ARC) Project in March 2026. This process evaluates potential impacts on wildlife, waterways, air quality, traffic and other factors. It will also include opportunities for public input throughout the process.
Scoping period for the environmental review
The Project’s environmental review process began with an initial comment period for agencies and the public to provide input on the scope of environmental issues to be addressed in the Project’s Environmental Impact Report (EIR) and Project alternatives to consider. The initial public comment period, regarding the scope and content of the EIR, closed at noon on April 13, 2026.
As part of the environmental review process, Marin Water held two evening public scoping meetings: Wed, March 25 at the Novato Recreation Center AND Thursday, March 26 at San Ramon Elementary School. The meetings provided opportunities for interested parties to learn more about the project and provide comments on the scope of environmental issues to be addressed in the Project’s EIR. The same information was provided on both meeting dates. Please see the following documents related to the scoping period.
- Notice of Preparation – Atmospheric River Capture (ARC) Project
- Initial Study Checklist – Atmospheric River Capture (ARC) Project
- Scoping Meeting Materials – includes a project elements map, pump station conceptual views, biological resources maps, proposed pipeline construction areas maps, and an overview of the CEQA review process and timeline
- Scoping Summary and Comments – This document describes the District’s CEQA scoping process and contains the comments received on the proposed project during the EIR scoping period. Comments received during the scoping process are part of the public record as documented in this scoping summary. The comments received during the public scoping process have been reviewed and considered by the District in determining the appropriate scope of issues to be addressed in the Draft EIR.
Drafting the EIR
As the CEQA Lead Agency, Marin Water will prepare the Project’s Environmental Impact Report (EIR) in compliance with CEQA. The draft EIR is currently expected to publish in summer or fall of 2026 and the next public comment period will open at that time.
Estimated Project 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 | 2030 |
*Timeline subject to change based on environmental review, easements and property acquisitions and permitting.
Frequently Asked Questions
About the Project
When would it operate?
Other than routine maintenance activities, the project would operate during dry years. These conditions generally include:
- Low reservoir storage levels in Marin
- Extended dry periods
- High flow in the Russian River from atmospheric river storm events
When these conditions align, the project can operate during key storm windows to store water for later use.
If the project relies on rain, how could it be helpful during dry periods?
Even in dry years, historical data shows that large winter storms can still produce significant flows in the Russian River watershed, which is about 40 times larger than the watershed feeding Marin Water’s seven local reservoirs.
These flows can remain elevated for weeks or even months, and much of this water still flows to the Pacific
Ocean, even after environmental requirements are met. The ARC Project would allow Marin Water to capture a limited portion of that water during these storm windows and store it for use later during extended dry periods.
Amount of water flowing to Pacific Ocean from Russian River
How would the project have worked in previous droughts?
Even in 1976-1977, the dryest year of record, approximately 27,000 acre-feet of excess Russian River water flowed out into the ocean, well above in-stream requirements. The ARC project would have enabled the District to capture and store some of this surplus water.
During the last drought in 2021, Nicasio Reservoir had enough available storage capacity that the ARC Project could have delivered over 16,000 acre-feet of additional water supply over a four-year period. At the time, the District was considering building an expensive emergency pipeline across the Richmond Bridge to ensure adequate water supply, which would not have been necessary had the ARC Project been in place.
Drought of Record – 1976-1977 (Winter months when ARC Project would operate)
What is the current estimated cost and how is the project being funded?
The current estimated cost is $195 million, with a potential range of $137 million to $293 million.
Marin Water is funding the design of the ARC Project through its capital budget program and plans to finance construction through bond funding. In parallel, the project team is also actively investigating all available state and federal funding opportunities that may help offset some of the project costs.
Project Status & Environmental Review
What environmental studies are being conducted?
Marin Water is currently preparing a draft Environmental Impact Report (EIR) in accordance with the California Environmental Quality Act (CEQA). The draft EIR will evaluate a range of environmental topics, including:
- Traffic and transportation
- Noise
- Biological resources (such as wetlands and oak woodlands)
- Cultural and archaeological resources
- Air quality and other environmental factors
Environmental surveys are also underway to better understand existing site conditions, including areas that may contain sensitive habitats or resources.
How will environmental impacts be addressed?
The draft EIR will identify potential environmental impacts of the project and evaluate ways to avoid,
minimize or mitigate those impacts where feasible. This includes consideration of construction-related impacts as well as operational effects.
Why ARC over other options like more storage tanks, increasing reservoir capacity, etc.?
Over the last five years, Marin Water has developed and evaluated a wide range of water supply and storage options as part of its long-term planning, including reservoir expansion, the creation of new reservoirs, desalination, recycled water, direct potable reuse, bolstered demand management efforts, and increased use of supplemental Russian River supplies. Based on that evaluation, the ARC Project was identified as the most practical and cost-effective near-term option for improving long-term water supply reliability.
The ARC Project would leverage existing infrastructure to create a connecting pipeline to access and store water during key storm events for use during dry periods.
As part of the EIR process now underway, a range of alternatives will continue to be formally evaluated and analyzed before any project approval is considered.
Working with the Community
How is Marin Water coordinating with other local agencies?
Marin Water is coordinating, and will continue to coordinate, with the City of Novato, County of Marin,
North Marin Water District, and other local agencies as the project is developed. This includes working together on construction planning to help minimize disruptions, as well as identifying opportunities, where feasible, to align the project with local priorities and provide added community value. As more detailed information becomes available, the District will continue to share updates and provide opportunities for public input, including during the draft EIR public review period.
Information for the Novato Community
Because portions of the proposed project would be located in and around the Novato city limits, these FAQs focus on what the project could mean for the community, including potential construction impacts and proposed facilities.
What to Expect in Novato
Where would construction occur in Novato?
Within Novato city limits, the proposed ARC Project would include approximately 3.5 miles of new pipeline, which would generally be constructed within or alongside existing roadways, including San Marin Drive and a short section of Novato Blvd near San Marin High School. A new pump station would also be constructed near Wood Hollow Drive in the San Marin area. The exact pipeline alignment and construction approach are still being refined and will be evaluated as part of the draft EIR.
What should residents expect during construction?
Construction will likely start in late 2027 – 2028. Work would move along the pipeline alignment over time, meaning impacts would be temporary and localized to specific segments rather than occurring along the full route at once. Because much of the pipeline would be installed within existing roadways, residents may experience temporary disruptions in areas where work is active. These could
include:
- Lane closures and traffic delays
- Noise and dust near active work zones
- Temporary parking restrictions
- Temporary access limitations near construction
Detailed construction impacts, including traffic and noise, will be evaluated as part of the draft Environmental Impact Report (EIR).
How would traffic and access be managed?
Maintaining access for residents, businesses, and emergency services is a key consideration of the
project. While detailed traffic control plans have not yet been finalized, the project will:
- Lane closures and traffic delays
- Noise and dust near active work zones
- Temporary parking restrictions
- Temporary access limitations near construction
Traffic management strategies will be further developed and evaluated as part of the EIR.
How long would construction take?
Pipeline construction within Novato is expected to be completed in roughly 14 to 18 months. Work would occur in segments, with active construction progressing along the pipeline alignment over time.
At a localized level, pipeline installation typically advances in segments of roughly 100 to 300 feet per
day, and similar cadence would be expected for ARC, meaning construction activity in any one area would be temporary.
A more detailed construction schedule will be developed as the project design advances.
Would construction near schools be addressed?
Yes. For areas near schools, such as San Marin High School, San Ramon Elementary and Pleasant Valley Elementary, construction timing would be planned to reduce potential conflicts where feasible. For example, work in these areas will be scheduled during summer months and other lower-traffic periods. Specific measures will be evaluated and refined as part of the draft EIR and in coordination with local agency partners in Novato.
What happens to roads in Novato after construction is complete?
Any roadway or surface disturbed during construction would be restored following completion of the work, in accordance with City of Novato requirements. Specific restoration approaches will be developed during final design and permitting, in coordination with the City. The project team will work with the City to ensure roadway conditions are restored to equal or better condition following construction.
Facilities in Your Area
Where would the pump stations be located?
The project proposes two new pump stations that would operate together to move water from the
existing aqueduct system to Nicasio Reservoir:
- Pump Station A – near Wood Hollow Drive and Redwood Blvd., close to US-101
- Pump Station B – near Stafford Lake
These locations are preliminary and will continue to be refined as part of the project design and environmental review process.
What would the pump stations look like?
Each pump station would include a building to house pumps, electrical equipment, and control systems. Facilities would be designed with consideration for the surrounding area, including architectural treatments intended to be compatible with community character. Final site layouts and designs are still being developed and will be shared with the community for input as they become available.
The Wood Hollow Drive (Pump Station A) site is expected to include a water storage tank. This tank would help regulate flows between the existing aqueduct system and the new pipeline. The size and configuration of the tank are still being determined and will be evaluated as part of the draft EIR.
Resources
- Technical Memorandum 3: Preferred Alternative Selection
- ARC project fact sheets
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
- Have a question about the project not covered here? Email arcproject@marinwater.org
- Sign up for ARC project email updates to be provided at key milestones by providing your email in the “subscribe” field below.