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 river 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 Gualala River was previously known as the Valhalla River, but the name was changed sometime between 1877 and 1921.