Hoover Dam, once known as Boulder Dam, is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the U.S. states of Arizona and Nevada. When completed in 1936, it was both the world's largest hydroelectric power generating station and the world's largest concrete structure. It was surpassed in both these respects by the Grand Coulee Dam in 1945. It is currently the world's 38th-largest hydroelectric generating station.
This dam, located 30 mi (48 km) southeast of Las Vegas, Nevada, is named after Herbert Hoover, who played an instrumental role in its construction, first as the Secretary of Commerce, and then later, as the President of the United States. Construction began in 1931, and was completed in 1936, a little more than two years ahead of schedule. The dam and the power plant are operated by the Bureau of Reclamation of the U.S. Department of the Interior. Listed on the National Register of Historic Places in 1981, Hoover Dam was designated a National Historic Landmark in 1985.
Lake Mead is the reservoir created by the dam, named after Elwood Mead, who oversaw the construction of the dam.
A commission was formed in 1922 with a representative from each of the Basin states and one from the Federal Government. The federal representative was Herbert Hoover, then Secretary of Commerce under President Warren Harding. In January 1922, Hoover met with the state governors of Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming to work out an equitable arrangement for apportioning the waters of the Colorado River for their states' use. The resulting Colorado River Compact, signed on November 24, 1922, split the river basin into upper and lower halves with the states within each region deciding how the water would be divided. This agreement, known as the Hoover Compromise, paved the way for the Boulder Dam Project. This huge dam was built to provide irrigation water flow, for flood control, and for hydroelectric-power generation.
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The first attempt to gain Congressional approval for construction of Boulder Dam came in 1922 with the introduction of two bills in the House of Representatives and the Senate. The bills were introduced by Congressman Phil D. Swing and Senator Hiram W. Johnson and were known as the Swing-Johnson bills. The bills failed to come up for a vote and were subsequently reintroduced several times. In December 1928, both the House and the Senate finally approved the bill and sent it to the President for approval. On December 21, 1928, President Calvin Coolidge signed the bill approving the Boulder Canyon Project. The initial appropriation for construction was made in July 1930, by which time Herbert Hoover had become President.
Early plans called for the dam to be built in Boulder Canyon, so the project was known as the Boulder Canyon Project. The dam site was eventually moved downstream eight miles (13 km) to Black Canyon, but the project name remained the same. A major motive for relocating to Black Canyon was that a dam at Boulder Canyon would not have provided sufficient physical control of the river below the damsite—the area of what is called the Boulder basin. Black Canyon provided a better total control of the river that far downstream.
The contract to build the Boulder Dam was awarded to Six Companies, Inc. on March 11, 1931, a joint venture of Morrison-Knudsen Company of Boise, Idaho; Utah Construction Company of Ogden, Utah; Pacific Bridge Company of Portland, Oregon; Henry J. Kaiser & W. A. Bechtel Company of Oakland, California; MacDonald & Kahn Ltd. of Los Angeles; and the J.F. Shea Company of Portland, Oregon. The chief executive of Six Companies, Frank Crowe, had previously invented many of the techniques used to build the dam.
During the concrete-pouring and curing portion of construction, it was necessary to circulate refrigerated water through tubes in the concrete. This was to remove the heat generated by the chemical reactions that solidify the concrete, since the setting and curing of the concrete was calculated to take about 125 years if cooling was not done. Six Companies, Inc., did much of this work, but it discovered that such a large refrigeration project was beyond its expertise. Hence, the Union Carbide Corporation was contracted to assist with the refrigeration needs.
Six Companies, Inc. was contracted to build a new town called Boulder City for workers, but the construction schedule for the dam was accelerated in order to create more jobs in response to the onset of the Great Depression, and the town was not ready when the first dam workers arrived at the site in early 1931. During the first summer of construction, workers and their families were housed in temporary camps like Ragtown while work on the town progressed. Discontent with Ragtown and dangerous working conditions at the dam site led to a strike on August 8, 1931. Six Companies responded by sending in strike-breakers with guns and clubs, and the strike was soon quelled. But the discontent prompted the authorities to speed up the construction of Boulder City, and by the spring of 1932 Ragtown had been deserted. Gambling, drinking alcohol, and prostitution were not permitted in Boulder City during the period of construction. To this day Boulder City is one of only two locations in Nevada not to allow gambling, and the sale of alcohol was illegal until 1969.
While working in the tunnels, many workers suffered from the carbon monoxide generated by the machinery there. The contractors claimed that the sickness was pneumonia and was not their responsibility. When Nevada officials tried to enforce state mining air-quality laws, the contractors took them to court. Officially, only 96 workers died constructing Hoover Dam. Some of the workers sickened and died because of the so-called "pneumonia". Most are uncounted on the official death list. [] "The Bureau of Reclamations fatality statistics show that 42 deaths were attributed to pneumonia during the construction period, more than any other cause." In January, 1936, the Six Companies made out-of-court settlements, in undisclosed amounts, with fifty gas-suit plaintiffs.
To protect the construction site from flooding, two cofferdams were constructed. Construction of the upper cofferdam began in September 1932, even though the river had not yet been diverted. A temporary horseshoe-shaped dike protected the cofferdam on the Nevada side of the river. After the Arizona tunnels were completed, and the river diverted, the work was completed much faster. Once the cofferdams were in place and the construction site dewatered, excavation for the dam foundation began. For the dam to rest on solid rock, it was necessary to remove all the riverbed's accumulated erosion soils and other loose materials until sound bedrock was reached. Work on the foundation excavations was completed in June 1933. During excavations for the foundation, approximately 1,500,000 cubic yards (1,150,000 m3) of material was removed. Since the dam would be a gravity-arch type, the side-walls of the canyon would also bear the force of the impounded lake. Therefore the side-walls were excavated too, to reach virgin (un-weathered) rock which had not experienced the weathering of centuries of water seepage, wintertime freeze cracking, and the heating/cooling cycles of the Arizona/Nevada desert.
To divert the river's flow around the construction site, four diversion tunnels were driven through the canyon walls, two on the Nevada side and two on the Arizona side. These tunnels were 56 feet (17 m) in diameter. Their combined length was nearly 16,000 ft (4,900 m) or more than 3 mi (4.8 km). Tunneling began at the lower portals of the Nevada tunnels in May 1931. Shortly afterwards, work began on two similar tunnels in the Arizona canyon wall. In March 1932, work began on lining the tunnels with concrete. First the base, or invert, was poured. Gantry cranes, running on rails through the entire length of each tunnel were used to place the concrete. The sidewalls were poured next. Movable sections of steel forms were used for the sidewalls. Finally, using pneumatic guns, the overheads were filled in. The concrete lining is 3 ft (0.91 m) thick, reducing the finished tunnel diameter to 50 ft (15 m).
Following the completion of the dam, the entrances to the two outer diversion tunnels were sealed at the opening and half way through the tunnels with large concrete plugs. The downstream halves of the tunnels following the inner plugs are now the main bodies of the spillway tunnels.
The two vertical foundations for each of the arch walls (the Nevada side and Arizona side) had to be founded on sound virgin rock; free of cracks and the weathering that the surface rock of the canyon walls had from thousands of years of weathering and exposure.
The men who removed this rock were called high-scalers. While suspended from the top of the canyon with ropes high-scalers climbed down the canyon walls and removed the loose rock with jackhammers and dynamite.
The first concrete was placed into the dam on June 6, 1933. Since no structure of the magnitude of the Hoover Dam had been constructed, many of the procedures used in construction of the dam were untried. Since concrete heats up and contracts as it cures, uneven cooling and contraction of the concrete posed a serious problem. The Bureau of Reclamation engineers calculated that if the dam were built in a single continuous pour, the concrete would have taken 125 years to cool to ambient temperature. The resulting stresses would have caused the dam to crack and crumble. To solve this problem the dam was built in a series of interlocking trapezoidal concrete pours. To further cool the concrete each form contained cooling coils of 1 inch (25 mm) thin-walled steel pipe. River water was circulated through these pipes to help dissipate the heat from the curing concrete. After this, chilled water from a refrigeration plant on the lower cofferdam was circulated through the coils to further cool the concrete. After each layer had sufficiently cooled, the cooling coils were cut off and pressure grouted by pneumatic grout guns. John L. Savage who supervised the design and construction of the dam was also responsible for developing the process used to cool the concrete.The concrete is still curing and gaining in strength as time goes on.
There is enough concrete in the dam to pave a two-lane highway from San Francisco to New York.
and the Mountain Time Zone]]
The initial plans for the finished facade of both the dam and the power plant consisted of a simple, unadorned wall of concrete topped with a Gothic-inspired balustrade and a powerhouse that looked like little more than an industrial warehouse.[] This initial design was criticized by many as being too plain and unremarkable for a project of such immense scale, so Los Angeles-based architect Gordon B. Kaufmann then the supervising architect to the Federal Bureau of Reclamation, headquartered in Denver was brought in to redesign the exteriors.[] Kaufmann greatly streamlined the buildings, and applied an elegant Art Deco style to the entire project, with sculptured turrets rising seamlessly from the dam face and clock faces on the intake towers set for the time in Nevada and Arizona, in the Pacific Standard Time Zone or Pacific Daylight Time Zone and Mountain Standard Time Zone time zones respectively (although because Arizona does not observe daylight saving time, the two clocks show the same time during the half of the year around the northern summer).
At Kaufmann’s behest, Denver artist Allen Tupper True was brought on board to handle the design and decoration of the walls and floors of the new dam. True conceived of incorporating motifs of the Navajo and Pueblo tribes of the region. Although some initially were opposed to these designs, True was given the go-ahead and was officially appointed Consulting Artist. With the assistance of the National Laboratory of Anthropology, True researched authentic decorative motifs from Indian sand paintings, weavings, baskets and ceramics. The images and colors are based on Native American visions of rain, lightning, water, clouds, local animals-lizards, serpents, birds-and on the Southwestern landscape of stepped mesas. In these works which are integrated into the walkways and interior halls of the great dam, True also reflected on the machinery of the operation making the symbolic patterns at once appear both ancient and modern.
These Native American motifs, embedded in the terrazzo floors, look like the cogs of the giant turbines, yet they are distinctly American Indian in origin. For True, the shapes and designs associated with the American Indians were akin to those of the ancient Greeks and Romans. True’s designs played well off Kaufmann’s monumental architecture and it could be said that together they created an American temple to modernity.
With the agreement of architect Kaufmann and the engineers, True also devised an innovative color coding for the pipes and machinery, which would be implemented throughout all the Federal Bureau of Reclamation projects. True’s Consulting Artist job lasted through 1942 and was extended so that he also completed design work for the Parker, Shasta and Grand Coulee dams and power plants. At the time, True’s work on the Boulder Dam was humorously noted in a poem which appeared in the New Yorker magazine, part of which read, “lose the spark, and justify the dream; but also worthy of remark will be the color scheme . . .”
There were 112 deaths associated with the construction of the dam. There are different accounts as to how many people died while working on the dam and who was the first and last to die. A popular story holds that the first person to die in the construction of Hoover Dam was J. G. Tierney, a surveyor who drowned while looking for an ideal spot for the dam. Coincidentally, his son, Patrick W. Tierney, was the last man to die working on the dam, 13 years to the day later. Ninety-six of the deaths occurred during construction at the site. However, another surveyor died prior to construction, while surveying a potential location for the dam, and these statistics do not include other incidental and coincidental (heat stroke, heart failure, etc.) deaths during construction.
A fleet of special dump cars were built by Six Companies for use on the railroad that aided construction. Today, one of these cars survives at the Western Pacific Railroad Museum at Portola, California. The Western Pacific Railroad acquired several of the cars following the end of construction and used them in company service.
side.]] Water flowing from Lake Mead through the gradually-narrowing penstocks to the powerhouse reaches a speed of about 85 mph (137 km/h) by the time it reaches the turbines. The entire flow of the Colorado River passes through the turbines (except for seepage around the edges of the dam through the semi-porous volcanic rock it rests against). The spillways are rarely used.
Following an uprating project from 1986 to 1993, the total gross power rating for the plant, including two 2.4 megawatt[] electric generators that power the plant's operations, is about 2080 megawatts.
Excavation for the powerhouse was carried out simultaneously with the excavation for the dam foundation and abutments. Excavation for the U-shaped structure located at the downstream toe of the dam was completed in late 1933 with the first concrete placed in November 1933.
Generators at the Dam's Hoover Powerplant began transmission of electricity from the Colorado River to Los Angeles, California 266 miles (428 km) away on October 26, 1936. Additional generating units were added through 1961. Original plans called for 16 large generators, 8 on each side of the river (see architectural illustrations) but two smaller generators were installed instead of one of the large ones on the Arizona side, for a total of 17. The smaller generators were used to serve smaller municipalities at a time when the output of each generator was dedicated to a municipality, before the dam's total power output was placed on the grid and made arbitrarily distributable.
Hydroelectric power plants have the ability to vary the amount of power generated, depending on the demand. Steam turbine power plants are not as easily throttled because of the amount of thermodynamic inertia contained in their systems.
Control of water was the primary concern in the building of the dam. Power generation allowed the dam project to be self sustaining: repaying the 50-year construction loan, and continuing to pay for the multi-million dollar yearly maintenance budget. Power is generated in step with and only with the release of water in response to downstream water demands.
The Bureau of Reclamation reports that the energy generated is allocated as follows:
| Area | Percentage |
|---|---|
| Metropolitan Water District of Southern California | 28.5393% |
| State of Nevada | 23.3706% |
| State of Arizona | 18.9527% |
| Los Angeles, California | 15.4229% |
| Southern California Edison Company | 5.5377% |
| Boulder City, Nevada | 1.7672% |
| Glendale, California | 1.5874% |
| Pasadena, California | 1.3629% |
| Anaheim, California | 1.1487% |
| Riverside, California | 0.8615% |
| Vernon, California | 0.6185% |
| Burbank, California | 0.5876% |
| Azusa, California | 0.1104% |
| Colton, California | 0.0884% |
| Banning, California | 0.0442% |
The dam is protected against overtopping by two spillways. The spillway entrances are located behind each dam abutment, running roughly parallel to the canyon walls. This spillway entrance arrangement forms a classic side-flow weir. Water flowing over the spillways drops sharply into spillway tunnels and connects to part of the construction diversion tunnels, and reenters the main river channel below the dam. This complex spillway entrance arrangement combined with the approximate 700 ft (210 m) elevation drop from the top of the reservoir to the river below is a difficult engineering problem and poses several design challenges. The overall spillway capacity was empirically verified in post construction tests in 1941. This test also showed that the spillway tunnels could be damaged from cavitation created by the high velocity flow when running near full volume. After further damage was incurred during use in six weeks in the summer of 1983, the tunnel linings were repaired and the spillway tunnel design was modified to minimize cavitation potential.
The large spillway tunnels have only been used three times in the history of the dam. In addition to use in 1941 and 1983, spillway use was required in 1999 when heavy precipitation in the watershed filled Lake Mead.
Hoover Dam and its associated changes in water use had devastating impact on the Colorado River Delta at the mouth of the Colorado River. The construction of the dam has been pointed to as the beginning of an era of decline of this estuarine ecosystem. For six years in the late 1930s, after the construction of the dam and while Lake Mead filled, virtually no flow of water reached the mouth of the river. The delta's estuary, which once had a freshwater-saltwater mixing zone stretching 65 kilometres (40 mi) south of the river's mouth, was turned into an inverse estuary where the level of salinity was actually higher closer to the river's mouth.
The Colorado River had experienced natural flooding before the construction of the Hoover Dam. The dam eliminated the natural flooding, which imperiled many species adapted to the flooding, including both plants and animals.
The construction of the dam decimated the populations of native fish in the river downstream from the dam. Four species of fish native to the Colorado River, the Bonytail chub, Colorado pikeminnow, Humpback chub, and Razorback sucker, are currently listed as endangered by the U.S. federal government.
There are two lanes for automobile traffic across the top of the dam. It serves as the Colorado River crossing for the highway U.S. Route 93. The two-lane section of road approaching the dam is narrow, has several dangerous hairpin turns, and is subject to rock slides.
provide much more highway capacity, and better safety, the new Hoover Dam Bypass is scheduled to be completed in 2010 and it will divert the U.S. 93 traffic 1,500 ft (460 m) downstream from the dam. The bypass will include a composite steel and concrete arch bridge, tentatively named the Mike O'Callaghan-Pat Tillman Memorial Bridge. Once the bypass is completed, through traffic will no longer be allowed across Hoover Dam.
Additionally, in the wake of the September 11, 2001 terrorist attacks there are significant security concerns. Because of the attack, the Hoover Dam Bypass project was expedited. Traffic across Hoover Dam is presently restricted. Some types of vehicles are inspected prior to crossing the dam while semi-trailer trucks, buses carrying luggage, and enclosed-box trucks over 40 feet (12 m) long are not allowed on the dam at all. That traffic is diverted south to a Colorado River bridge at Laughlin, Nevada.
The dam, originally planned for a location in Boulder Canyon, was relocated to Black Canyon for better impoundment, but was still known as the Boulder Dam project. The Boulder Canyon Project Act of 1928 (BCPA) never mentions a proposed name or title for the dam. The BCPA merely allows the government to "...construct, operate, and maintain a dam and incidental works in the main stream of the Colorado River at Black Canyon or Boulder Canyon..." Work on the project started on July 7, 1930.
At the official beginning of the project on September 17, 1930, President Hoover's Secretary of the Interior Ray Lyman Wilbur, announced that the new dam on the Colorado River would be named Hoover Dam to honor the then President of the United States. Wilbur followed a standing tradition of naming important dams after the President who was in office when they were constructed, such as the Theodore Roosevelt Dam, the Wilson Dam, and the Coolidge Dam. However, these dams were not named after sitting presidents, as they were completed after the presidents had left office. More tellingly, Hoover was already campaigning for re-election in the face of the Depression and he sought credit for creating jobs. A Congressional Act of February 14, 1931, made the name "Hoover Dam" official.
However, in 1932, Herbert Hoover lost his bid for reelection to Franklin Delano Roosevelt. In his memoirs, Hoover wrote of stopping to inspect progress on the dam, by night, on November 12, 1932, on his way back to Washington from his Palo Alto, California, home after his defeat. He commented, "It does give me extraordinary pleasure to see the great dream I have so long held taking form in actual reality of stone and cement. It is now ten years since I became chairman of the Colorado River Commission.... This dam is the greatest engineering work of its character ever attempted by the hand of man." He went on to list its purposes, concluding, "I hope to be present at its final completion as a bystander. Even so I shall feel a special personal satisfaction." (Hoover adds a footnote to this, see below.)
When Roosevelt took office on March 4, 1933, he brought Harold Ickes with him to replace Ray Lyman Wilbur as Secretary of the Interior. Ickes wasted no time removing Hoover’s name from the Boulder Canyon Project. On May 8, 1933, Ickes issued a memorandum to the Bureau of Reclamation, which was in charge of the dam, stating, "I have your reference to the text for the pamphlet descriptive of the Boulder Canyon Project for use at the Century of Progress Exposition. I would be glad if you will refer to the dam as 'Boulder Dam' in this pamphlet as well as in correspondence and other references to the dam as you may have occasion to make in the future."
This did not happen immediately, but over the following several years all references to Hoover Dam in official sources, as well as tourist and other promotional materials, vanished in favor of Boulder Dam.
Roosevelt died in 1945 and Harold Ickes retired in 1946. On March 4, 1947 California Republican Congressman Jack Anderson submitted House Resolution 140 to restore the name Hoover Dam. Anderson’s resolution passed the House on March 6; a companion resolution passed the Senate on April 23, and on April 30, 1947, President Harry S. Truman signed Public Law 43 which read: "Resolved … that the name of Hoover Dam is hereby restored to the dam on the Colorado River in Black Canyon constructed under the authority of the Boulder Canyon Project Act … . Any law, regulation, document, or record of the United States in which such dam is designated or referred to under the name of Boulder Dam shall be held to refer to such dam under and by the name of Hoover Dam."
Hoover writes this footnote to his comments of November 12, 1932: "Responding to a suggestion from Hiram Johnson, and with his characteristic attitude, Secretary Ickes changed the name of the dam. The hint in the above address that I should like to be present did not secure me an invitation to the dedication ceremonies conducted by President Roosevelt. I have never regarded the name as important. The important thing is a gigantic engineering accomplishment that will bring happiness to millions of people." In 1947, the Congress, by practically unanimous action, restored the name Hoover Dam — to Mr. Ickes' intense indignation."
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