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Supply and Demand Assessment (SDA) – Tomales-Drakes Bay

  Overview

The Tomales – Drakes Bay Watershed spans approximately 220 square miles in western Marin County, with a small portion extending into Sonoma County. The watershed’s core feature, Tomales Bay, is a 12-mile-long, shallow coastal estuary less than a mile wide, bordered by the San Andreas Fault zone and fed by two primary tributaries—Lagunitas Creek and Walker Creek—along with about 20 smaller intermittent streams. Lagunitas Creek and its tributaries account for roughly 52% of the watershed’s drainage area, while Walker Creek covers about 35%. The region features a wide range of ecological zones, including redwood forests, coastal scrub, freshwater marshes, and intertidal wetlands. It supports a rich diversity of aquatic and terrestrial species, including endangered coho salmon, steelhead trout, tidewater goby, and California freshwater shrimp. Much of the watershed remains in agricultural use—particularly dairy and grazing operations—which cover more than half of the land area.

Water quality concerns in the watershed stem from nonpoint source pollution such as livestock runoff, failing septic systems, and sedimentation from unpaved roads. These have led to shellfish bed closures, swimming advisories, and elevated mercury levels in fish. The Tomales Bay Watershed Council and partners developed the Integrated Coastal Watershed Management Plan (ICWMP) to address these challenges. The plan emphasizes protecting ecological integrity, supporting mariculture and agriculture, improving water quality, and enhancing water supply reliability. Management strategies are built on collaboration among local communities, water districts, landowners, agencies, and conservation groups, all working to safeguard the watershed’s ecological and economic values.

Tomales-Drakes Bay Watershed

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  Announcements

May 15, 2025 - Tomales – Drakes Bay Watershed Model Work Plan - Now Available
The work plan from Paradigm Environmental for development of a hydrologic model for the Tomales–Drakes Bay Watershed is now publicly available in Model Development section below. Please email questions regarding the model development to DWR-SDA@waterboards.ca.gov.

  Outreach and Engagement

  Modeling in the Drakes Bay Watershed

The SDA Unit has continued to work on the models and tools developed for the Russian River Watershed during the State Water Board's 2021-2022 Regional Drought Response. These tools provided the necessary information for the Division of Water Rights (Division) to assess water availability and issue water right curtailments to ensure adequate, minimal water supplies for critical purposes under the emergency regulation.

To capture the water demand in the SDA Program Watersheds, water right demand estimates were incorporated based on information from annual reports of water diversion and use submitted to the Division. The SDA Unit continued to develop the Division’s demand dataset methodology and converted it into a series of R scripts and manual review spreadsheets that apply data quality correction flags. For example, the scripts identified and corrected unit conversion errors, duplicate reporting, and misplotted points of diversion. The SDA Unit then uses a water allocation tool adopted from the UC Davis Drought Water Rights Allocation Tool (DWRAT) to allocate available supply amongst water right holders based on their respective water right priority and water demands. This version of DWRAT allocates water at the subbasin level and accounts for imported flows.

The SDA Unit’s demand methodology, along with the supply scripts and the watershed specific versions of DWRAT, will be made available on the California Water Board Data Center GitHub page as separate repositories.

  Stay Informed

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Contact Us

If you have questions or please email them to: DWR-SDA@waterboards.ca.gov