Auburn Dam was a proposed concrete arch dam on the North Fork of the American River east of the town of Auburn, California, in the United States, on the border of Placer and El Dorado Counties. Slated to be completed in the 1970s by the U.S. Bureau of Reclamation, it would have been the tallest concrete dam in California and one of the tallest in the United States, at a height of and storing of water. Straddling a gorge downstream of the confluence of the North and Middle Forks of the American River and upstream of Folsom Lake, it would have regulated water flow and provided flood control in the American River basin as part of Reclamation's immense Central Valley Project.
The dam was first proposed in the 1950s; construction work commenced in 1968, involving the diversion of the North Fork American River through a tunnel and the construction of a massive earthen cofferdam. Following a nearby earthquake and the discovery of an unrelated seismic fault that underlay the dam site, work on the project was halted for fears that the dam's design would not allow it to survive a major quake on the same fault zone. Although the dam was redesigned and a new proposal submitted by 1980, spiraling costs and limited economic justification put an end to the project until severe flooding in 1986 briefly renewed interest in Auburn's flood control potential. The California State Water Resources Control Board denied water rights for the dam project in 2008 due to lack of construction progress.
Although new proposals surfaced from time to time after the 1980s, the dam was never built for a number of reasons, including limited water storage capacity, geologic hazards, and potential harm to recreation and the local environment. Much of the original groundwork at the Auburn Dam site still exists, and up to 2007, the North Fork American River still flowed through the diversion tunnel that had been constructed in preparation for the dam. Reclamation and Placer County Water Agency completed a pump station project that year which blocked the tunnel, returned the river to its original channel, and diverted a small amount of water through another tunnel under Auburn to meet local needs. However, some groups continued to support construction of the dam, which they stated would provide important water regulation and flood protection.
Background Starting in the 1850s during the California Gold Rush, the city of Sacramento rapidly grew around the confluence of the Sacramento River and its tributary the American River, near the middle of the Central Valley of California. The city's increasing population necessitated the construction of an extensive system of levees on the two rivers to prevent flooding. These early flood control works were insufficient; in 1862, the city was inundated so completely that the state government was temporarily moved to San Francisco. In 1955, the U.S. Army Corps of Engineers built the Folsom Dam at the confluence of the North and South Forks of the American River to provide flood control for the Sacramento metropolitan area. However, the Folsom Dam, with a capacity of just compared to the annual American River flow of , proved inadequate. A flood in 1955 filled the Folsom Reservoir to capacity, before the dam was completed; and it has filled many times since.Smith, p. 29 However, increased water uses and diversions, requirements for 200-year flood control, and joint system operations have increased seasonal flood capacity in Folsom Lake.USACE 2015, p. 340
The demand for irrigation water in the Sacramento area and other parts of the Central Valley were also growing. In 1854, a diversion dam was constructed on the North Fork American River at the site of Auburn Dam, to divert water into ditches that supplied downstream farms. Irrigation with dam and canal systems was favored because the seasonal nature of the American River caused floods in some years and droughts in others.Smith, p. 28 A large dam at the Auburn site was thus considered for both flood control and water supply. In the 1950s, the Bureau of Reclamation created the first plans for a high dam at Auburn. Several designs, ranging from earth-fill to concrete gravity dams, were considered. Before the dam could be built, the Auburn-Foresthill Road – which crosses the river just upstream of the dam site – had to be relocated. Even before the project was authorized, contracts were let for the construction of a high bridge to carry the road over the proposed reservoir, as well as preliminary excavations at the dam site.
The eventual design of Auburn Dam called for the creation of a reservoir with of capacity, more than twice that of Folsom Lake. The extra storage would greatly reduce the flood risk to Sacramento. The dam was to be the principal feature of the Auburn-Folsom South Unit of the Central Valley Project, with the purpose to "provide new and supplemental water for irrigation, municipal and industrial use, and to replenish severely depleted ground water in the Folsom South region". Congress authorized the project in 1965; the targeted completion date was 1973.
As the Auburn Dam proposal evolved, the project transformed from a primary flood-control structure to a multipurpose high dam that would serve various other purposes including long-term water storage, hydroelectricity generation, and recreation. One of the first ideas, publicized in the late 1950s, called for a embankment dam impounding of water. In 1963, a earthfill dam holding back of water was proposed. The pre-construction design was finalized in 1967, for a concrete thin-arch gravity structure over high. This dam would be long, thick at the base, and equipped with five generators at its base for a total generating capacity of . Two concrete-lined flip bucket spillways would abut both sides of the dam. With the initial plans set and the project authorized, construction work for the dam started in late 1968.
Construction
Site preparation
Official groundbreaking of the Auburn Dam started on October 19, 1968, with preparatory excavations and test shafts drilled into the sides of the North Fork American River gorge. The contract for the diversion tunnel through the mountainside on river left, in diameter, long, and equipped to handle a flow of (a roughly 35-year flood) was let to Walsh Western for about $5.1 million in 1968. The actual construction of the tunnel itself did not begin until mid-1971, and it was completed in late November 1972. One worker was killed during the excavation of the tunnel. In 1975, the earthen cofferdam for the Auburn project, high, was completed, diverting the river into the tunnel. The diversion tunnel bypassed a roughly section of the riverbed to allow construction of the main dam.
Upstream of the dam site, Auburn-Foresthill Road – one of the only all-weather thoroughfares of the region – would be inundated by the proposed reservoir. In preparation for the reservoir's filling, it was rerouted over a three-span, truss bridge rising above the river. Even though Auburn Dam would never be completed, the bridge was still required because the pool behind the cofferdam would flood the original river crossing.Smith, pp. 29–30 It also improved safety and reduced travel time by eliminating a steep, narrow and winding grade into the canyon on either side of the river, as comparisons to maps showing the old road alignment will attest. The contracts for various projects pertaining to the relocation of the roadway were given to O.K. Mittry and Sons, Hensel Phelps Construction Company, and Willamette-Western Corporation, the latter for the construction of the actual bridge. The Foresthill Bridge, the fourth highest bridge in the United States, was completed in 1973.
Earthquake and redesigning In 1975, a magnitude 5.7 earthquake shook the Sierra Nevada near Oroville Dam, approximately north of the Auburn Dam construction site. This quake concerned geologists and engineers working on the project so much that the Auburn Dam construction was halted while the site was resurveyed and investigations conducted into the origins of the earthquake. It was discovered that the quake might have been caused by reservoir-induced seismicity, i.e. the weight of the water from Lake Oroville, whose dam had been completed in 1968, was pressing down on the fault zone enough to cause geologic stress, during which the fault might slip and cause an earthquake. As the concrete thin-arch design of the Auburn Dam could be vulnerable to such a quake, the project had to be drastically redesigned.
Over the next few years, while all construction was stayed, Reclamation conducted evaluations of the seismic potential of the dam site, even though these delays caused the cost of the project to rise with every passing year. The studies concluded that a major fault system underlay the vicinity of the Auburn Dam site, with many folds of metamorphic rock formed by the contact of the foothill rocks and the granite batholith of the Sierra Nevada. Reclamation predicted that the Auburn Reservoir could induce an earthquake of up to a 6.5, while the U.S. Geological Survey projected a higher magnitude of 7.0. Nevertheless, Reclamation redesigned the Auburn Dam based on their 6.5 figure, even though a 7.0 would be three times stronger. The design for the Auburn Dam was changed to a concrete thick-arch gravity dam, to provide better protection against a possible earthquake induced by its own reservoir.
Through the rest of the 1970s, other possible designs were looked at but never implemented, while preliminary work on the construction site resumed. On April 29, 1979, the foundations for the Auburn Dam were completed. However, debates continued over whether to build an arched or straight-axis gravity dam. Some favored the latter design because it would have greater mass, allowing it to better withstand earthquakes.
Cofferdam failure In early February 1986 of rain fell on the Sacramento region in 11 days, melting the Sierra Nevada snowpack and causing a huge flood to pour down the American River. The 1986 floods were some of the most severe recorded in the 20th century; Placer County was quickly designated a Federal Disaster Area. Rampaging streams and rivers incurred some $7.5 million in damages within the county. The rating for Sacramento's levees, supposedly designed to prevent a 125-year flood, was dropped to a 78-year flood in studies conducted after the 1986 event, which suggested that such weather occurred more frequently than previously believed. The floods tore out levees along the Sacramento and Feather Rivers through the Sacramento Valley, and the city of Sacramento was spared by a close margin. Folsom Lake filled to dangerously high levels with runoff from the North, Middle and South Forks of the American River.