Nicasio Reservoir in 2026: The Withdrawn Spillway Plan and the Separate ARC Pipeline Proposal
By Matthew Martinez, Founder, CEO & Managing Broker, Diamond Real Estate Group. California DRE #01869679.
Two different Marin Water proposals can become confused because both involve Nicasio Reservoir. One would have raised the amount of water the reservoir could hold. The other would create a way to move winter water into existing storage.
As of October 28, 2026, their status is also different. Marin Water’s dated update says the spillway-enlargement proposal stopped in 2025. The separate Atmospheric River Capture project remains in preliminary design and environmental review.
The rubber-gate spillway proposal was withdrawn
The Nicasio Spillway Modification project page includes an August 21, 2025 update stating that the District was no longer actively pursuing the project. It was withdrawn from environmental review and removed from active planning.
The page retains the former proposal’s description: an inflatable gate across Seeger Dam’s spillway intended to add approximately 3,700 acre-feet of storage. That retained background should be read together with the later status update.
The District cites higher anticipated costs, potential reductions in environmental yield and regulatory, ecological and social complexities among the reasons for stopping the work. Those are the agency’s stated project findings, rather than a conclusion about every possible future reservoir project.
ARC proposes a connection to existing storage
The Atmospheric River Capture project page describes a proposed 13.2-mile pipeline connecting the North Marin Aqueduct with Nicasio Reservoir, with pumping facilities and dechlorination.
The District states that ARC would not increase Nicasio Reservoir’s existing storage capacity. Its purpose is to store available winter water conveyed through the Russian River supply system for later dry-period use.
This is the central distinction: increasing the size of a container and improving the ability to fill available space are separate kinds of infrastructure work.
The two projects at a glance
| Proposal | Intended change | Documented status |
|---|---|---|
| Nicasio spillway rubber gate | Increase reservoir storage capacity | Stopped and withdrawn from review in 2025 |
| Atmospheric River Capture pipeline | Convey winter supply into existing storage | Preliminary design and environmental review in 2026 |
A shared reservoir name does not make the projects interchangeable. Their physical work, operating purpose and review history need to remain separate when discussing the Nicasio landscape.
The 2026 ARC review expanded to include creek management
The District’s ARC page records a September 18, 2026 Supplemental Notice of Preparation adding a long-term management plan for sediment and vegetation in Upper Nicasio Creek. Its supplemental comment period closed October 19, 2026.
The page describes the Environmental Impact Report as being prepared and gives an estimated future timeline subject to review, acquisitions and permits. These are planning milestones, rather than evidence that the pipeline is operating.
The Water Supply Roadmap places ARC within a broader portfolio of conservation, conveyance and system improvements developed after the 2020–2022 drought.
The September supplemental notice itself identifies work in both Upper Nicasio Creek and Halleck Creek, with initial sediment and vegetation removal followed by periodic maintenance. It also identifies a lowered sediment bench within the reservoir.
Those details show that the revised review concerns a wider physical scope than a pipeline alone. The creek-management component has its own locations, work sequence and maintenance implications within the proposed project.
Why the distinction matters locally
Nicasio’s reservoir landscape is both a familiar local setting and part of a regional water system. Conversations about proposed changes become clearer when the actual project and its latest dated action are identified.
The two records illustrate why a project description alone can be incomplete. An agency may preserve an older concept for reference after deciding to stop it. Reading the status update alongside the physical proposal gives a more accurate account of what is happening around Nicasio.
Explore real estate and home for sale in Nicasio, or contact Diamond to discuss a location that fits your plans.
Sources checked October 28, 2026. Historical details are attributed to the linked primary records and site operators. Check the operator for current visiting arrangements.
