The City of Santa Barbara completed construction of Mission Tunnel in 1912 and Gibraltar Dam in 1920, and thus accomplished the first diversion of water from the Santa Ynez River Basin to the South Coast area. Mission Tunnel, about 3.7 miles in length, was designed to intercept groundwater flow and to later convey water from Gibraltar Reservoir to the City of Santa Barbara. Infiltration into Mission Tunnel varies with rainfall, but averages approximately 1,100 AFY. Gibraltar Dam construction began in 1914 and was completed in 1920. During the construction of the dam and reservoir, the City's water supply became so deficient that residents had to revert to the use of well water, and even that use was restricted.
In the winter of 1920-21, the first rainy season after the completion of the dam, the reservoir failed to fill because the rainfall was below average. In the 1921-22 season, with rainfall only slightly above average, the reservoir filled to capacity, and a large volume went over the spillway, causing extensive damage.
By 1945, sedimentation had reduced storage in Gibraltar Reservoir from 14,500 AF to approximately 7,800 AF. In 1948, the dam was raised 23 feet and storage capacity was restored to approximately the original volume. However, sedimentation has continued to decrease the storage capacity of the reservoir by an average of 150 AFY. Visit the City of Santa Barbara's website for more information.