High Quality Content by WIKIPEDIA articles! The Mattole River is a river on the north coast of California, that flows northerly, then westerly into the Pacific Ocean. A short section of the river flows through upper Mendocino County, but the vast majority of its 62-mile (100 km) run is through southern Humboldt County. Communities, from north to south, closely associated with the Mattole River include: Petrolia, Honeydew, Ettersburg, Thorn Junction, and Whitethorn. The river enters the ocean at the Mattole Estuary about 4 miles west-southwest of Petrolia and 10 miles south of Cape Mendocino.