The Gualala River is a river on the northern coast of California. Most of the river is in Sonoma County, but a portion is in Mendocino County. The headwaters of the 40-mile-long (64 km) river (measuring via its South Fork) are high in the Coast Range, and it empties into the Pacific Ocean. For its last few miles, it forms the boundary between Sonoma County and Mendocino County.The river has three forks: the South Fork, Wheatfield Fork and the North Fork.