"...Old Mission Dam, whose associated aqueduct and flume extended about 5 miles to the Mission of San Diego de Alcalá, was one of the first major irrigation engineering projects on the Pacific coast of the United States. It impounded water from the San Diego River, and for much of the year provided an assured supply, which was released as needed for agricultural and domestic purposes...."
A NATIONAL HISTORIC LANDMARK
Information from NPS.gov's website