This river drains a long, narrow basin of in extreme southern California, with its mouth less than north of the United States-Mexico border. Much of the river's watershed is mountainous, and the highest point is above sea level at Stonewall Peak in the Cuyamacas Mountains. A large portion of the drainage lies within the Cleveland National Forest, but few trees are actually supported by the arid climate. The Sweetwater is the largest river flowing into San Diego Bay.