The San Jacinto River is a 42-mile-long river in Riverside County, California. The river's headwaters are in Santa Rosa and San Jacinto Mountains National Monument. The lower portion of the 765-square-mile watershed is urban and agricultural land. As a partially endorheic watershed that is contiguous with other Great Basin watersheds, the …The San Jacinto River is a 42-mile-long river in Riverside County, California. The river's headwaters are in Santa Rosa and San Jacinto Mountains National Monument. The lower portion of the 765-square-mile watershed is urban and agricultural land. As a partially endorheic watershed that is contiguous with other Great Basin watersheds, the western side of the San Jacinto Basin is a portion of the Great Basin Divide.