Supply and Demand Assessment (SDA) – Mattole River

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.