Gibraltar Dam is located on the Santa Ynez River in southeastern Santa Barbara County, California, in the United States. It forms Gibraltar Reservoir and is owned by the city of Santa Barbara. Originally built in 1920 and expanded in 1948, the dam and reservoir are located in a remote area of the Los Padres National Forest.
The main purpose of Gibraltar Dam is to provide water for homes and daily use. It supplies about 4,600 acre-feet (5,700,000 m³) of water to Santa Barbara each year, meeting nearly 30% of the city’s needs. Water diverted from the dam also helps power a small hydroelectric plant. Due to sediment buildup, the reservoir has lost much of its capacity. The lake often fills and overflows after a single storm but may completely dry up in some years. As of February 2019, sedimentation had reduced the reservoir’s capacity to 4,314 acre-feet (5,321,000 m³), which is only 19% of its original design.
The dam is built in a section of the Santa Ynez River called the "Gibraltar Narrows," which is also the name of the Gibraltar (or Sunbird) mercury mine. This mine operated near the current location of Gibraltar Reservoir from the 1870s until the 1990s.
Description
Gibraltar Dam is a circular concrete arch dam that is 194.5 feet (59.3 meters) high and 600 feet (180 meters) long. It is located about 72 miles (116 kilometers) from the mouth of the Santa Ynez River and just above where Devils Canyon meets the river. The dam manages water flow from an area covering 216 square miles (560 square kilometers) with an annual water supply of 24,000 acre-feet (30,000,000 cubic meters). The Santa Ynez River area upstream is mostly wilderness, and water flow is not controlled except by the smaller Juncal Dam, which belongs to the Montecito Water District. Below Gibraltar Dam, the Santa Ynez River flows into Lake Cachuma, a much larger reservoir owned by the U.S. Bureau of Reclamation.
A concrete spillway extends south from the main dam and is controlled by four manually operated radial gates. During non-flood seasons, flashboards can be placed on top of the gates to increase the reservoir’s storage space. The spillway can handle about 90,000 cubic feet per second (2,500 cubic meters per second) of water. The spillway is designed so that overflow water flows over the natural sandstone cliff next to the dam, forming a waterfall that is 150 feet (46 meters) high.
The original storage capacity of Gibraltar Reservoir was 15,374 acre-feet (18,964,000 cubic meters). After an expansion in 1948, the total storage capacity increased to 22,500 acre-feet (27,800,000 cubic meters). However, sediment buildup reduced the usable storage to about 15,000 acre-feet (19,000,000 cubic meters). Since then, the dam has not been raised, and sediment has not been removed, causing the reservoir’s usable capacity to drop to one-third of its original size. As of February 2019, sediment had further reduced the reservoir’s capacity to 4,314 acre-feet (5,321,000 cubic meters), or 19% of its designed capacity.
Water is taken from the reservoir through an intake structure near the southern end of the dam and sent through the 3.7-mile (6.0-kilometer) Mission Tunnel. This tunnel carries water under the Santa Ynez Mountains to a small regulation basin called Lauro Reservoir. The water is either treated at the Cater Water Treatment Plant before being sent to the city’s water system or released into Mission Creek to help replenish groundwater. The Mission Tunnel can handle 40 cubic feet per second (1.1 cubic meters per second) of water.
At the end of the Mission Tunnel is the 820 kilowatt Lauro hydroelectric plant. It was built in 1985 but stopped operating in 1998 because of rising costs. The city fixed and restarted the plant in 2015 after federal licensing costs decreased, but drought conditions prevented the plant from operating until early 2017.
Background and construction
In the early 1900s, the City of Santa Barbara ran out of local water supplies and looked to the Santa Ynez River, which has a much larger drainage area than the city's local streams. In 1904, construction began on the Mission Tunnel, which was dug through Mission Canyon in the Santa Ynez Mountains to access surface and groundwater in the Santa Ynez basin. At that time, it was the longest water tunnel in the world. The city planned to build a dam on the Santa Ynez River to increase the water supply for the Mission Tunnel. Between 1913 and 1919, the city issued revenue bonds totaling $820,000 to fund the dam and water system.
Several dam sites were considered, including locations near Mono Creek, Juncal, and the Main River. However, these were not chosen, and Gibraltar was selected because it offered the best storage capacity, highest water flow, and the most stable ground for building the dam. Engineers studied different designs, including rock-fill, masonry, and concrete. The Mission Tunnel was extended to the Gibraltar site, allowing water to flow by gravity to Santa Barbara. Work on the site began in 1913, starting with building foundations in the riverbed. A $40,000 contract was given to Arthur S. Bent Construction Co. for this task. In 1917, a concrete "thrust block" was built on the canyon's south side to support the future dam, as the rock there was not tall enough. The dam contract was awarded to Bent Brothers and W.A. Kraner on July 8, 1918.
Because there was no road to the site, workers and supplies were moved through an 18-inch (460 mm) gauge electric railway inside the Mission Tunnel. The tunnel was only 4 feet (1.2 m) high and 3.5 feet (1.1 m) wide, limiting the size of equipment used. Machines like steam shovels, concrete mixers, and rock crushers had to be taken apart and moved piece by piece through the tunnel. Up to three trains operated at once, with a siding in the middle of the tunnel allowing supplies to be delivered every half-hour. Dynamite and other dangerous materials could not be taken inside the tunnel and had to be carried over the Santa Ynez Mountains on a rough trail. Water leaking from the tunnel roof created constant dripping, and care was taken to prevent contamination, as this water eventually flowed into Santa Barbara’s drinking supply.
After solving these challenges, construction of the dam began quickly. In the first three months, 15,000 cubic yards (11,000 m³) of concrete were poured. A 4-by-6-foot (1.2 m × 1.8 m) hole was left in the dam’s base to allow river water to flow through during construction. A total of 270,000 sacks of cement and 400,000 board feet of lumber were transported through the tunnel. Rock and gravel for concrete were mined from the riverbed and processed at a nearby plant. Concrete was placed on the dam using a cableway system attached to a 185-foot (56 m) steel tower. Construction paused for three months during winter 1918–1919 due to flooding risks in the gravel mining area. The dam was initially built 170 feet (52 m) above the riverbed and 185 feet (56 m) above bedrock.
The dam was completed on January 23, 1920, becoming the first structure to hold back water from the Santa Ynez River. Due to the remote location, the project cost nearly $2,000,000. According to the Engineering News-Record (1920), the dam’s alignment was precise, with no deviation greater than one inch. The hole in the dam’s base was sealed with a temporary gate before being filled with concrete from downstream, allowing the reservoir to begin filling. However, the city lacked funds to finish the spillway. A dry winter in 1920–1921 prevented the reservoir from filling, forcing the city to use groundwater. Heavy rains in January and February 1922 filled the reservoir for the first time, but flooding damaged the temporary spillway. Erosion near an unknown fault zone beneath the spillway nearly caused it to collapse. The spillway was rebuilt at a cost of $90,000.
Gin Chow v. Santa Barbara
In 1928, Gin Chow, a farmer who owned land near the Santa Ynez River, took legal action against the city of Santa Barbara. He argued that the city did not have the right to take water from Gibraltar Reservoir. Soon after, 39 other farmers in the Santa Ynez Valley joined the case. Chow’s argument was based on riparian water rights, a rule that gives landowners along a river the right to use the full, unbroken flow of water passing through their property, even if they do not use it. This rule was challenged by a new state law passed in 1928, which stated that water must be used in a way that benefits the public and that water rights do not allow wasting or unreasonable use of water. The Gin Chow case became the first major legal test of this new law.
Five years later, in 1933, the California Supreme Court ruled in favor of the city of Santa Barbara. The court decided that the water being taken from the river was stormwater, not part of the river’s regular flow. This meant the city’s actions did not harm landowners downstream, as the stormwater would have naturally flowed into the ocean anyway. The court allowed the city to take up to 4,189 acre-feet of water each year from Gibraltar Dam as a "prescriptive right." However, the city was required to release at least 616 acre-feet of water during late summer and fall, when the river’s natural flow is lowest. The city was also allowed to take extra water from heavy rainstorms. These rules remain in place today.
The Gin Chow case is considered a key moment in California water law. It supported the 1928 state law, which limited the power of riparian landowners to control water resources. This change allowed cities, like Santa Barbara, to claim rights to surface water as long as they followed the principle of "reasonable use." The court stated that what counts as reasonable use depends on the specific details of each case. It also noted that California’s growing population would increasingly rely on stored water, rather than letting it flow unused into the ocean. This decision set a legal example that helped the state build more dams.
Expansion projects
After the reservoir was built, it quickly faced problems with heavy sediment buildup, which became worse because of occasional wildfires in the areas above the dam. In 1932, the Matilija Fire burned about 50,000 acres (20,000 hectares) of the drainage basin. To reduce erosion from the burned land, the U.S. Forest Service built two debris dams upstream. The Mono Debris Dam, an Ambursen-type structure, was completed in 1936 and filled completely with 1 million cubic yards (770,000 cubic meters) of sediment by 1938. The Agua Caliente Debris Dam, a concrete-arch structure, was finished in late 1937 and filled with 750,000 cubic yards (570,000 cubic meters) of sediment by 1941. These projects only slightly extended the reservoir’s useful life. The March 1938 flood caused a large amount of debris to flow downstream, resulting in $340,000 in damage in the Lompoc area.
By 1948, sediment had filled half of the reservoir. A severe drought in the winter of 1947–1948 drained the remaining water, leading to an emergency water rationing rule. In summer 1948, the dam was raised by 23 feet (7.0 meters), and a new buttressed spillway section was added. This restored the reservoir’s capacity to 14,000 acre-feet (17,000,000 cubic meters) when it filled again in the next winter. The $1.1 million cost for the expansion was covered by bonds issued by the city of Santa Barbara. However, the reservoir continued to fill with sediment faster than expected, leading to more frequent water releases and reduced water availability for the city. This situation influenced the federal government to build Bradbury Dam in 1953 to store more water from the Santa Ynez River in Lake Cachuma, about 25 miles (40 kilometers) downstream of Gibraltar Dam.
In 1983, a seismic evaluation found Gibraltar Dam at risk of failure during a strong earthquake. The U.S. Army Corps of Engineers rehabilitated the dam by adding a buttress of roller-compacted concrete to the downstream side. Although this project did not increase storage capacity, it allowed for a potential 20-foot (6.1-meter) height increase in the future if needed. The work, completed between October and December 1990, used 93,000 cubic yards (71,000 cubic meters) of concrete. This change transformed the dam from a pure arch structure to an arch-gravity structure. The total cost of the project was $8.18 million.
"Pass-through" operation and future plans
In the early 1980s, the city of Santa Barbara wanted to raise the height of Gibraltar Dam. However, the California Department of Water Resources and the U.S. Bureau of Reclamation said that making the dam larger would harm the flow of water downstream into Lake Cachuma reservoir. In 1989, an agreement called the "Upper Santa Ynez River Operations Agreement" or "Pass Through Agreement" was created. This agreement allowed some water that would normally spill from Gibraltar Dam to be legally stored in Lake Cachuma for the city’s use. This change made it unnecessary to raise the dam.
In 2017, the U.S. Bureau of Reclamation estimated that about 210 acre-feet (260,000 cubic meters) of sediment entered the reservoir each year. This amount is about 1.3% of the dam’s original design capacity. In the 21st century, the reservoir has often had low water levels because of long-term droughts and high water use. In 2016, the reservoir dried up completely, and water had to be transported by truck to the dam keeper.
As of 2017, there were no plans to remove sediment from the reservoir or reduce the amount of sediment entering it. The 1989 agreement assumed that when the reservoir became nearly full of sediment, it would operate as a run-of-the-river project, which means it would only move water instead of storing it. Most storage functions would then be handled by Lake Cachuma. However, the Bureau of Reclamation predicted that the reservoir would not fill completely with sediment. Instead, it would stabilize at about 2,000 acre-feet (2,500,000 cubic meters) of water due to high water flows washing sediment out. This level is expected to be reached around 2031.
Environmental issues
The Gibraltar Dam blocked steelhead fish from reaching the upper part of the Santa Ynez River when it was completed in 1920. In 1916, the California Fish and Game Commission suggested building a fish ladder with the dam to help fish move past it, but no such structure was ever built. The dam changed how water flows downstream, the movement of dirt and sand, the quality of stream habitats, and the water quality. It also delayed steelhead migration during early rainy seasons by holding back floodwaters that might have helped fish move past a sandbar at the river’s mouth.
No reliable numbers are known about the steelhead population before the dam was built, but it is believed to have been much larger. The dam did not seem to greatly affect steelhead populations downstream. In 1946, scientists described the habitat between Solvang and Gibraltar Dam as "excellent." Steelhead migration to the base of Gibraltar Dam ended when Bradbury Dam was completed in 1953. In 2004, a plan was proposed to move adult steelhead from below Bradbury Dam to the river above Gibraltar Dam so they could complete their migration naturally, but this plan was never carried out.
Although the dam disrupted some aquatic habitats, it also created new land near the river as sediment filled the upstream part of Gibraltar Reservoir. A wide floodplain with a clear stream channel formed, and this area now supports typical California riparian woodland, including willow, cottonwood, and oak trees. This habitat helps endangered species like the least Bell’s vireo.
The 2007 Zaca Fire burned much of the upper Santa Ynez watershed, sending large amounts of ash and dirt into Gibraltar Reservoir. Chemicals in the ash mixed with chlorine used to treat Santa Barbara’s water supply and created harmful substances. Cleaning the water cost the city nearly $3 million. More recent fires, such as the 2016 Rey Fire and the 2017 Thomas Fire, also affected the Gibraltar watershed.
Concerns about mercury contamination in the reservoir have been raised due to the nearby old Gibraltar Mine (Sunbird Mine), which stopped operating in 1991. However, no mercury has been found in the reservoir, and no cases of mercury poisoning in Santa Barbara County have been linked to the mine.
Public access
Although the dam and reservoir are open to the public, only official vehicles are allowed on the narrow, winding dirt roads that lead to the dam. The shortest way to reach the dam on foot is a 6-mile (9.7 km) round trip hike along the Santa Ynez River from Red Rock Campground. The dam can also be reached from several trails that start at Camino Cielo Road. The Gibraltar Trail runs along the southern edge of the reservoir from the dam, and other trails branch out around the reservoir into the backcountry of Los Padres National Forest. Boating, fishing, and swimming are not allowed at the reservoir because it is a public water supply.