Cottonwood Creek, originally Arroyo Alamos, a tributary stream of San Luis Creek, in Merced County, California. Its mouth, before the San Luis Dam was built across the course of San Luis Creek, originally was at its confluence with San Luis Creek at at approximately 250 feet / 76 meters. Today its mouth is located in the foothills of the Diablo Range, where Cottonwood Creek enters the north side of San Luis Reservoir at an elevation 548 feet / 167 meters. The source of the creek is at at an elevation of 2080 feet / 634 meters on the east slope of Red Hill , a summit on the eastern slope of the Diablo Range.