O'Shaughnessy Dam (California)
|Location||Tuolumne County, California, United States|
|Construction began||August 1, 1919|
|Opening date||May 24, 1923|
|Owner(s)||San Francisco PUC|
|Dam and spillways|
|Type of dam||Concrete arch gravity|
|Height||430 ft (130 m)|
|Length||900 ft (270 m)|
|Base width||308 ft (94 m)|
|Crest elevation||3,812 ft (1,162 m)|
|Creates||Hetch Hetchy Reservoir|
|Capacity||360,400 acre·ft (444,500,000 m3)|
|Catchment area||459 sq mi (1,190 km2)|
|Surface area||1,972 acres (798.0 ha)|
|Turbines||3 x Pelton turbines at Kirkwood Powerhouse
2 x Pelton turbines at Moccasin Powerhouse
|Installed capacity||234 MW|
|Annual generation||976 GWh|
O'Shaughnessy Dam is a 430 ft (130 m) high concrete arch-gravity dam in Tuolumne County, California, in the United States. The dam impounds the Tuolumne River at the lower end of Hetch Hetchy Valley in Yosemite National Park, about 160 mi (260 km) east of San Francisco. The dam and reservoir serves as the primary water source for the Hetch Hetchy Project, which provides municipal water in the San Francisco Bay Area. Construction of the dam started in 1919 and was finished in 1923, with the first water delivered in 1934. The dam is named for engineer Michael O'Shaughnessy, who oversaw its construction.
Since its initial proposal, the dam has been highly controversial for its flooding of the scenic Hetch Hetchy Valley, once compared to Yosemite Valley for its natural beauty. The debate over the dam continues today, with preservationists contending for its removal and restoration of the local environment, while others argue that leaving the dam in place would be a better decision both economically and environmentally.
In the late 19th century, the city of San Francisco was rapidly outgrowing its limited water supply, which depended on intermittent local springs and streams. The city looked east to the Sierra Nevada, where snowmelt fed the headwaters for many of California's largest rivers. In 1890, San Francisco mayor James D. Phelan proposed to build a dam and aqueduct on the Tuolumne River, one of the largest southern Sierra rivers, as a way to increase and stabilize the city's water supply. In 1900, a United States Geological Survey (USGS) report also described the Tuolumne River as "the best source of sustainable water for San Francisco". Although Phelan managed to secure water rights for the Tuolumne River in 1901, his appeals to the federal government for development of the Hetch Hetchy Valley were unsuccessful. But when 1906 San Francisco earthquake and subsequent fire razed the city, the dangerous inadequacies of its water supply system were brought to national light.
Out of fourteen potential water sources considered by the city, which included Lake Tahoe, the Eel River, and tributaries of the Sacramento and San Joaquin Rivers, Hetch Hetchy was considered superior for its excellent dam site, abundant sediment-free water, lower cost and hydroelectric potential. At the time, Hetch Hetchy was an isolated, seldom visited subalpine valley, visited intermittently by gold seekers and sheepherders. However, one major hurdle stood in the way: since 1890, Hetch Hetchy Valley and the surrounding lands had been protected by Yosemite National Park and were off-limits to utility development, let alone at the grand scale proposed by the city. Even though the valley was not well known to the general public, preservationist groups such as the Sierra Club treasured it for its spectacular beauty, often compared to that of Yosemite Valley itself. Led by naturalist and mountaineer John Muir, the Sierra Club adamantly opposed the city of San Francisco as it sought permission from the federal government to build a dam in the valley.
San Francisco jumped this hurdle in 1908 when Secretary of the Interior James R. Garfield responded to its appeal, granting the city rights to development at Hetch Hetchy, stating that "Hetch Hetchy was not unique, a lake would be even more beautiful than its meadow floor and the hydroelectric power generated could eventually pay for the costs of construction." One of the strongest supporters of the Hetch Hetchy project was Gifford Pinchot, Chief Forester of the United States Forest Service, who pushed a policy of "conservation through use" which promoted the sustainable development of natural resources in the U.S. On December 19, 1913, President Woodrow Wilson signed the Raker Act, which permitted San Francisco's development of the Hetch Hetchy project on the terms that water and power derived from the project could only be used for public utilities, not private profit. The rationale was that since Hetch Hetchy lay on public land, it was reasonable for its natural resources to be developed for the public benefit. Though highly controversial, the bill passed Congress by a vote of 43 for and 25 against.
Muir and the preservationists were outraged by the federal government's permission for San Francisco to develop the Hetch Hetchy Valley. However, on December 24, 1914, with construction on the dam barely underway, Muir died, leaving his Sierra Club to fight a protracted battle against the Hetch Hetchy Project over the next ten years. "Dam Hetch Hetchy!" Muir had said – "As well dam for water tanks the peoples' cathedrals and churches, for no holier temple has been consecrated by the heart of man!" The Sierra Club argued that it was not necessary for San Francisco to destroy the valley for its water supply, pointing out the availability of other sites with reasonable proximity – including the Mokelumne River, which the U.S. Army Corps of Engineers reported in 1913 as "a better and cheaper source than Hetch Hetchy". (The Mokelumne was later dammed, in a similar scheme to the Hetch Hetchy project, to provide water to the East Bay.) By this point, however, the city had become "obsessed" with developing Hetch Hetchy, and "dismissed or discarded other rivers and valleys that would have served them better… as if it was created for their purpose."
Work on the Hetch Hetchy project began in early 1914 shortly after the passage of the Raker Act. The city hired John R. Freeman, who had previously worked on the water supply systems of Boston and New York City, to plan the complex dam and aqueduct system. Civil engineer Michael O'Shaughnessy would oversee the construction and design details of the Hetch Hetchy project. The dam in Hetch Hetchy Valley would subsequently be named in his honor. Before construction of O'Shaughnessy Dam could commence, the city completed a 70 ft (21 m) high dam at Lake Eleanor to provide water for the Early Intake Powerhouse, which was necessary to provide electricity for the construction site of the larger dam.
Initial construction of the dam cost $7.4 million, which was paid for largely by bonds issued by the city of San Francisco. To transport workers and materials, the city hired Frederick Rolandi to oversee construction the Hetch Hetchy Railroad, a 68 mi (109 km) standard gauge line that followed the narrow canyon of the Tuolumne River to the remote dam site. Built from 1915 to 1918 by a workforce of roughly 900, the railway allowed for supplies to be shipped directly from San Francisco along the Southern Pacific and Sierra lines which connected to the Hetch Hetchy line. The railroad principally used geared Shay locomotives to negotiate its dangerous winding curves and steep 4 percent grades.
Actual groundbreaking on O'Shaughnessy Dam was on August 1, 1919, when Utah Construction Company of San Francisco began preparing the dam site for construction. Workers began clearing the trees in Hetch Hetchy Valley to prepare it for receiving waters of the future reservoir. A 20 ft (6.1 m) diameter tunnel, later expanded to 23 × 25 ft (7.0 × 7.6 m), was dug around the south side of the O'Shaughnessy Dam site, and a timber crib cofferdam diverted the waters of the Tuolumne River into the tunnel during construction. The riverbed on the site of the future dam was excavated over 100 ft (30 m) before hitting the granite bedrock. A retaining wall was poured on the upstream side to prevent water seepage into the foundation hole, and the granite was scoured and artificially roughened to prepare for receiving concrete.
The concrete for the dam was processed in a plant located shortly upstream from the construction site, with sand and rock excavated from abundant alluvial deposits in the Hetch Hetchy valley. This was mixed with cement shipped in on the Hetch Hetchy Railroad and local boulders ranging from 1 ft (0.30 m) to several yards (metres) in diameter to produce a cyclopean construction material for the dam. Beginning in September 1921, concrete was poured by chutes from a 375 ft (114 m) tower on the south side of the gorge. A total of 398,516 cu yd (304,687 m3) of concrete was poured to form a dam standing 226 ft (69 m) above the riverbed and 344 ft (105 m) above foundations. The last concrete was placed in February 1922 and the dam was completed in May 1923. At the time, it was the second tallest dam in the United States, after Idaho's Arrowrock Dam. On May 24, 1923, the reservoir filled for the first time. Thousands of people worked on the dam during its construction, which claimed the lives of 67 men and one woman.
The first hydropower was delivered in 1925 with the completion of the Moccasin Powerhouse, fed by Hetch Hetchy water through the Canyon and Mountain Tunnels. However, the first water deliveries did not reach San Francisco until 1934, eleven years after the completion of O'Shaughnessy Dam and twenty years after groundbreaking of the Hetch Hetchy project. O'Shaughnessy Dam had been designed with foundations and a unique stepped face to accommodate a future raise, in anticipation of rapid growth in water and hydroelectricity demand. Indeed, between 1935 and 1938, the dam was raised by 85 ft (26 m); a new spillway and outlet channels were constructed to accommodate the increased height and storage capacity, which helped to increase summer generation at downstream powerhouses.
The dam and reservoir today
Completed to its final dimensions in 1938, O'Shaughnessy Dam stands 312 ft (95 m) above the riverbed and 430 ft (130 m) above bedrock. The crest spans 900 ft (270 m) with a 17 ft (5.2 m) wide roadway crossing the top; the thickness of the dam wall reaches a maximum 308 ft (94 m) at the base. Altogether, the structure contains 662,605 cu yd (506,598 m3) of concrete and 700,000 lb (320,000 kg) of steel. Aside from normal water flows through the Canyon Tunnel to the Hetch Hetchy Aqueduct, water is released from the reservoir through eleven jet-flow gates on the dam face and an unlined side spillway controlled by three 65 ft (20 m) wide steel drum gates. With gates lowered, the spillway has a capacity of 48,600 cu ft/s (1,380 m3/s). Behind the dam, Hetch Hetchy Reservoir stretches for 8 mi (13 km) along the Tuolumne River, submerging Hetch Hetchy Valley and the lowermost section of the Grand Canyon of the Tuolumne. At maximum capacity, the reservoir stores 360,400 acre·ft (444,500,000 m3), covering 1,972 acres (798.0 ha). The dam and reservoir receive water from the upper 459 sq mi (1,190 km2) of the Tuolumne River watershed, and are supplied with water by Falls Creek, Tiltill Creek, and Rancheria Creek in addition to the main stem of the Tuolumne.
Hetch Hetchy water drives turbines in the Kirkwood and Moccasin Powerhouses located downstream along the Tuolumne River. Kirkwood Powerhouse came online in 1967 with two Pelton units, with a third added in 1987, bringing the total generating capacity to 124 megawatts (MW). Kirkwood is serviced with a hydraulic head of 1,450 ft (440 m) through the Canyon Tunnel, and produces an annual average of 549 million kilowatt hours (KWh). A new powerhouse was built to replace the old Moccasin Powerhouse in 1969. The new Moccasin Powerhouse, located near Lake Don Pedro lower on the Tuolumne River, has a capacity of 110 MW from two Pelton turbines. Moccasin generates 427 million KWh per year, and is fed by Hetch Hetchy water through the Mountain Tunnel, which provides a maximum head of 1,300 ft (400 m).
Water diverted at O'Shaughnessy Dam feeds into the Hetch Hetchy Aqueduct, which provides 85 percent of the municipal water for 2.4 million people in the San Francisco Bay Area. Because of the unique geology of the Hetch Hetchy watershed, which consists of shallow soils underlain by solid granite bedrock, water that flows into the reservoir is exceptionally clear and of very high quality. This quality is further maintained by stringent protection of the watershed; boating and swimming are prohibited at Hetch Hetchy Reservoir (although fishing is permitted at the reservoir and in the rivers which feed it). As a result, San Francisco tap water is some of the cleanest in the United States, without even the need for filtration, and is said to be of better quality than most bottled water.
O'Shaughnessy Dam has been controversial since its original construction both for its environmental impact and violations of the Raker Act by the city of San Francisco. Although the Raker Act explicitly stated that power and water from the Hetch Hetchy Project could be used only for public purposes, San Francisco has sold Hetch Hetchy power to Pacific Gas & Electric (PG&E) since 1925. Dam removal advocates have repeatedly pointed out that San Francisco's utilization of Yosemite National Park for water and power is unfair because of the damage to tourism and the local environment caused by the dam and reservoir. By draining the reservoir, removing the dam and restoring the valley to its original state, visitors to the park would once again be able to enjoy the natural beauty of Hetch Hetchy – once compared to that of Yosemite Valley – as it rightfully should have been protected under the national park. There are many rivers and reservoirs available for San Francisco to replace the water supply, such as the city's currently unused share of water in Lake Don Pedro, the biggest reservoir on the Tuolumne River.
Opponents of dam removal state that the estimated demolition cost of $3–10 billion is a poor investment, especially in regards of the resulting loss of renewable hydroelectric power, which would have to be replaced by polluting fossil fuel generation. Although there are many options available to replace San Francisco's water, none are of the purity currently supplied by Hetch Hetchy. There is also no guarantee that the valley can be successfully restored, as the original valley floor was actually the product of thousands of years of intensive controlled burning and management by Native American tribes that once lived in the area. Without this intervention, a forest would grow in the place of the valley's renowned meadows. Finally, the increased pressure of new tourism could cause its own environmental damage, as has been demonstrated in the crowded Yosemite Valley.
Despite the hotly contested status of O'Shaughnessy Dam in the environmental field, and occasional federal money set aside for studying alternatives to Hetch Hetchy – such as $7 million provided by President George W. Bush in 2007 in the National Park Service budget – public support for its removal is still relatively low. In 2012, San Francisco voters struck down Proposition F, which would have ordered the city to study the removal of O'Shaughnessy Dam and draft a plan to replace Hetch Hetchy water by 2016, by a vote of 77 percent against.
- List of largest reservoirs of California
- List of power stations in California
- List of the tallest dams in the United States
- Pulgas Water Temple
- San Francisco Public Utilities Commission
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|Wikimedia Commons has media related to Hetch Hetchy.|
- Historic images of the dam during and after construction
- Daily storage and release data for O'Shaughnessy Dam and Hetch Hetchy Reservoir – California Department of Water Resources