Gibraltar Dam is located on the Santa Ynez River, in southeastern Santa Barbara County, California, in the United States. Forming Gibraltar Reservoir, the dam is owned by the city of Santa Barbara. Originally constructed in 1920 and expanded in 1948, the dam and reservoir are located in a remote part of the Los Padres National Forest.
The main purpose of Gibraltar Dam is domestic water supply. It provides about of water to Santa Barbara each year, supplying almost 30% of the city's needs. Water diverted from the dam also powers a small hydroelectric plant. Due to having lost a massive portion of its capacity to sediment build-up, the lake can often fill and spill after a single storm, while drying up completely in some years. As of February 2019, sedimentation has reduced the reservoir's capacity to , only 19% of its designed capacity.
The dam is built in a part of the Santa Ynez River called the "Gibraltar Narrows" that gave its name to the Gibraltar (or Sunbird) mercury mine, which operated next to what is now Gibraltar Reservoir between the 1870s and 1990s.
Description
Gibraltar Dam is a constant radius concrete arch dam high and long. The dam is located about from the mouth of the Santa Ynez River and just above the confluence of Devils Canyon with the river. The dam controls runoff from a drainage basin of with an annual inflow of . The Santa Ynez basin upstream is mostly wilderness and inflows are unregulated with the exception of the smaller Juncal Dam (owned by the Montecito Water District). Downstream of Gibraltar, the Santa Ynez flows into the much larger Lake Cachuma, owned by the U.S. Bureau of Reclamation.
The concrete spillway extends southward from the main dam and is controlled by four manually operated radial gates. Flashboards can be installed atop the gates during the non-flood season to increase the reservoir capacity. The spillway capacity is about . The spillway is designed so that overflow water cascades down the natural sandstone face adjacent to the dam, creating a high waterfall.
The original design capacity of Gibraltar Reservoir was . After its 1948 expansion the gross capacity was increased to , though so much sediment had already accumulated that the usable capacity was still only about . Since then, the dam has not been raised nor any sediment dredged from the reservoir, and it has gradually declined to a third of its original capacity. As of February 2019, sedimentation has reduced the reservoir's capacity to , or only 19% of its designed capacity.
Water is drawn from the reservoir via an intake structure near the south end of the dam into the Mission Tunnel, which transports water beneath the Santa Ynez Mountains to a small regulation basin, Lauro Reservoir. The water is either treated at the Cater Water Treatment Plant before entering the city's distribution system, or released into Mission Creek for groundwater recharge. The Mission Tunnel has a diversion capacity of .
At the end of the Mission Tunnel is the 820 kilowatt Lauro hydroelectric plant, which was initially constructed in 1985 but was idled in 1998 due to increasing costs. The city repaired and recommissioned the plant in 2015 after the cost of federal licensing decreased, but drought prevented the plant from actually running until early 2017.
Climate
Background and construction
In the early 1900s the City of Santa Barbara had exhausted its local water supplies, and looked to the drainage basin of the nearby Santa Ynez River, which is much larger than any of the city's local streams. In 1904 construction began on the Mission Tunnel which was bored under Mission Canyon in the Santa Ynez Mountains, to tap surface and ground water in the Santa Ynez basin. At that time, it was the longest water tunnel in the world. The city sought to build a dam somewhere on the Santa Ynez River in order to increase the water supply available for Mission Tunnel. Between 1913 and 1919, revenue bonds totaling $820,000 were issued for the construction of the dam and water system.