Old Mission Dam, Dam in Mission Trails Regional Park, San Diego County, US.

Old Mission Dam spans the San Diego River in Mission Trails Regional Park, about 5 miles (8 km) northeast of Mission San Diego de Alcalá. Built of stone and cement, it was originally 220 feet (67 m) long, 13 feet (4 m) wide at its base and 12 feet (3.7 m) high. Behind it the dam held water released in controlled flows when the seasonal river dried out in summer.

Spanish colonists and members of the Kumeyaay Nation built this structure in 1803 to supply water to Mission San Diego de Alcala during drought periods. It became essential infrastructure for the mission's survival and continued in use for many generations.

The dam shows early water management engineering developed by Spanish colonists working alongside Kumeyaay laborers. It stands as a symbol of colonial infrastructure adapted to handle the region's dry climate.

The dam sits in the hills about 5 miles (8 km) northeast of Mission San Diego de Alcalá, reached via the Father Junipero Serra Trail from Mission Trails Regional Park. The San Diego River here was seasonal and dried out in summer, so carry water and wear walking shoes on hot days.

The dam's original dimensions were 220 feet long, 13 feet wide at the base and 12 feet high. It was the first major colonial-era irrigation project on the Pacific coast of the United States. The surviving remnant was designated a National Historic Landmark in 1963.