Gibraltar Dam and reservoir are located on the Santa Ynez River in Santa Barbara County, about 9 miles north of the City and upstream from Lake Cachuma . The City owns and operates the dam and reservoir pursuant to a Notice of Appropriation posted on October 11, 1904. Stored water is diverted through Mission Tunnel to the Cater Water Treatment Plant. The dam is a constant radius, concrete arch dam constructed in 1920 with an original capacity of 14,500 AF; it was raised to current elevation in 1949 and strengthened in 1991.Gibraltar Reservoir is the source of about one-third of the City of Santa Barbara’s water supply. Loss of storage capacity due to siltation has been an issue since the dam was constructed. To monitor changes in capacity, and pursuant to the requirements of the Upper Santa Ynez River Operations Agreement, the City commissions a bathymetric survey of Gibraltar Reservoir at approximately three year intervals.
|Watershed Drainage Area||216 square miles|
|Water Surface Elevation at Normal Spill||1,400 ft above sea level|
|Annual Yield||Average of approximately 4,600 acre feet per year|
|Current Capacity||5,251 acre feet at 1,400 ft elevation (per 2010 bathymetric survey)|
Diversions are limited by the 1930 Gin Chow legal judgment and the 1989 Upper Santa Ynez River Operations Agreement (USYROA, also known as the "Pass Through Agreement"). The USYROA was developed to resolve concerns that the City’s planned raising of Gibraltar Dam would impact the feasibility of a potential Cachuma enlargement project. The City agreed to defer the planned raising of Gibraltar Dam in exchange for the right to “pass through” some of its Gibraltar water to Lake Cachuma for delivery to the City through Tecolote Tunnel. The amount of Pass Through water is based on the difference in Gibraltar spills under actual conditions as compared to a hypothetical “Base Reservoir.” The Base Reservoir is equal in size to the 1988 reservoir and “operated” (by computer model) according to a compromise interpretation of the Gin Chow judgment agreed to as a part of the USYROA. A basic goal of the agreement is to allow the City to stabilize the yield from Gibraltar at approximately 1988 levels while minimizing impacts on the Cachuma Project and other downstream interests.
Costs for this source of supply are primarily "sunk" or fixed costs, including the original cost of construction, plus a cost of $9 million for strengthening in 1990-91, plus the cost of Mission Tunnel. Variable costs for Gibraltar water consist of the marginal cost of treatment at Cater Treatment Plant which is approximately $100/AF.