Pacoima Wash, 33 miles (53 km) long, is a major tributary of the Tujunga Wash, itself a tributary of the Los Angeles River, in the San Fernando Valley of Los Angeles County, California.The stream begins at the Pacoima Dam Reservoir in the western San Gabriel Mountains of the Angeles National Forest and proceeds south in a free-flowing stream alongside Pacoima Trail Road. From there, it joins several other unnamed streams that drain the nearby mountains, collecting at Lopez Dam.