Supply and Demand Assessment (SDA) – Mattole River
Overview
The Mattole River watershed spans approximately 304 square miles in southern Humboldt and northern Mendocino counties, draining from the rugged King Range (elevation of about 1200 ft) to the Pacific Ocean near Petrolia (sea level). This remote coastal basin features steep terrain, active fault lines, and intense winter precipitation. The Mattole River flows freely for 62 miles without dams, joined by 74 perennial tributaries before reaching the ocean. The watershed is dominated by evergreen forest, shrubland, and grasslands. The watershed also provides critical habitat for threatened anadromous fish, including Chinook and coho salmon and steelhead trout. These populations have experienced steep declines mainly due to increase in the river sediment load and increase instream temperatures.
Ongoing restoration and community engagement efforts have made the Mattole River a model for watershed-scale recovery. Multiple nonprofit and agency partners—including the Mattole Restoration Council, Mattole Salmon Group, and Sanctuary Forest—are working collaboratively to improve instream flows, reduce sediment loads, and enhance spawning and rearing habitat. The watershed was listed as impaired for stream sediment and temperature in 2002, and restoration efforts continue under a regional sediment TMDL framework. In addition, the State Water Board is supporting long-term planning through development of a hydrologic model to assess water supply and demand under varying climate and diversion scenarios. These efforts will support both ecological objectives and the needs of rural communities that depend on the watershed’s limited surface water availability.
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Announcements
May 15, 2025 - Mattole River Watershed Model Work Plan - Now Available
The work plan from Paradigm Environmental for development of a hydrologic model for the Mattole River watershed is now publicly available in Model Development section below. Please email questions regarding the model development to DWR-SDA@waterboards.ca.gov.
Model Development
Outreach and Engagement
Modeling in the Mattole River 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.
Additional Resources
Supply and Demand Assessment Program Links
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Contact Us
If you have questions or please email them to: DWR-SDA@waterboards.ca.gov
