Cottonwood Creek is a southeastward-flowing 4 mi (6.4 km) tributary stream of San Luis Creek,[3] originating in the eastern foothills of the Diablo Range in Merced County, California.

Today, Cottonwood Creek enters San Luis Reservoir, which raises the elevation of the creek's mouth to 548 ft (167 m), where it becomes Cottonwood Bay on the north side of the reservoir.

The source of the creek is at Red Hill, a summit on the eastern slope of the Diablo Range.

[4] Cottonwood Creek, originally named by the Spanish Arroyo Alamos, was named for the Fremont cottonwood (Populus fremontii) trees which are still abundant along the lower creek.

This area is now home to a herd of tule elk, visible from the Pacheco Pass (Highway 152), which dispersed there from 1978 to 1981 CDFW translocations of the large ungulates to the Hewlett-Packard San Felipe Ranch on Mt.