 = Hoover Dam = 
 
 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 Nevada and Arizona . It was constructed between 1931 and 1936 during the Great Depression and was dedicated on September 30 , 1935 , by President Franklin D. Roosevelt . Its construction was the result of a massive effort involving thousands of workers , and cost over one hundred lives . The dam was controversially named after President Herbert Hoover . 
 Since about 1900 , the Black Canyon and nearby Boulder Canyon had been investigated for their potential to support a dam that would control floods , provide irrigation water and produce hydroelectric power . In 1928 , Congress authorized the project . The winning bid to build the dam was submitted by a consortium called Six Companies , Inc . , which began construction on the dam in early 1931 . Such a large concrete structure had never been built before , and some of the techniques were unproven . The <unk> summer weather and lack of facilities near the site also presented difficulties . Nevertheless , Six Companies turned over the dam to the federal government on March 1 , 1936 , more than two years ahead of schedule . 
 Hoover Dam <unk> Lake Mead , the largest reservoir in the United States by volume . The dam is located near Boulder City , Nevada , a municipality originally constructed for workers on the construction project , about 30 mi ( 48 km ) southeast of Las Vegas , Nevada . The dam 's generators provide power for public and private utilities in Nevada , Arizona , and California . Hoover Dam is a major tourist attraction ; nearly a million people tour the dam each year . The heavily travelled U.S. 93 ran along the dam 's crest until October 2010 , when the Hoover Dam Bypass opened . 
 
 = = Background = = 
 
 
 = = = Search for resources = = = 
 
 As the United States developed the Southwest , the Colorado River was seen as a potential source of irrigation water . An initial attempt at diverting the river for irrigation purposes occurred in the late 1890s , when land speculator William Beatty built the <unk> Canal just north of the Mexican border ; the canal dipped into Mexico before running to a desolate area Beatty named the Imperial Valley . Though water from the Imperial Canal allowed for the widespread settlement of the valley , the canal proved expensive to maintain . After a catastrophic breach that caused the Colorado River to fill the <unk> Sea , the Southern Pacific Railroad spent $ 3 million in 1906 – 07 to stabilize the waterway , an amount it hoped <unk> would be <unk> by the Federal Government . Even after the waterway was stabilized , it proved unsatisfactory because of constant disputes with landowners on the Mexican side of the border . 
 As the technology of electric power transmission improved , the Lower Colorado was considered for its hydroelectric @-@ power potential . In 1902 , the Edison Electric Company of Los Angeles surveyed the river in the hope of building a 40 @-@ foot ( 12 m ) rock dam which could generate 10 @,@ 000 horsepower ( 7 @,@ 500 kW ) . However , at the time , the limit of transmission of electric power was 80 miles ( 130 km ) , and there were few customers ( mostly mines ) within that limit . Edison allowed land options it held on the river to lapse — including an option for what became the site of Hoover Dam . 
 In the following years , the Bureau of Reclamation ( BOR ) , known as the Reclamation Service at the time , also considered the Lower Colorado as the site for a dam . Service chief Arthur Powell Davis proposed using dynamite to collapse the walls of Boulder Canyon , 20 miles ( 32 km ) north of the eventual dam site , into the river . The river would carry off the smaller pieces of debris , and a dam would be built incorporating the remaining rubble . In 1922 , after considering it for several years , the Reclamation Service finally rejected the proposal , citing doubts about the unproven technique and questions as to whether it would in fact save money . 
 
 = = = Planning and agreements = = = 
 
 In 1922 , the Reclamation Service presented a report calling for the development of a dam on the Colorado River for flood control and electric power generation . The report was principally authored by Davis , and was called the Fall @-@ Davis report after Interior Secretary Albert Fall . The Fall @-@ Davis report cited use of the Colorado River as a federal concern , because the river 's basin covered several states , and the river eventually entered Mexico . Though the Fall @-@ Davis report called for a dam " at or near Boulder Canyon " , the Reclamation Service ( which was renamed the Bureau of Reclamation the following year ) found that canyon unsuitable . One potential site at Boulder Canyon was <unk> by a geologic fault ; two others were so narrow there was no space for a construction camp at the bottom of the canyon or for a spillway . The Service investigated Black Canyon and found it ideal ; a railway could be laid from the railhead in Las Vegas to the top of the dam site . Despite the site change , the dam project was referred to as the " Boulder Canyon Project " . 
 With little guidance on water allocation from the Supreme Court , proponents of the dam feared endless litigation . A Colorado attorney proposed that the seven states which fell within the river 's basin ( California , Nevada , Arizona , Utah , New Mexico , Colorado and Wyoming ) form an <unk> compact , with the approval of Congress . Such <unk> were authorized by Article I of the United States Constitution but had never been concluded among more than two states . In 1922 , representatives of seven states met with then @-@ Secretary of Commerce Herbert Hoover . Initial talks produced no result , but when the Supreme Court handed down the Wyoming v. Colorado decision <unk> the claims of the upstream states , they became anxious to reach an agreement . The resulting Colorado River Compact was signed on November 24 , 1922 . 
 Legislation to authorize the dam was introduced repeatedly by Representative Phil Swing ( R @-@ <unk> . ) and Senator <unk> Johnson ( R @-@ <unk> . ) , but representatives from other parts of the country considered the project as hugely expensive and one that would mostly benefit California . The 1927 Mississippi flood made Midwestern and Southern <unk> and <unk> more sympathetic toward the dam project . On March 12 , 1928 , the failure of the St. Francis Dam , constructed by the city of Los Angeles , caused a disastrous flood that killed up to 600 people . As that dam was a curved @-@ gravity type , similar in design to the arch @-@ gravity as was proposed for the Black Canyon dam , opponents claimed that the Black Canyon dam 's safety could not be guaranteed . Congress authorized a board of engineers to review plans for the proposed dam . The Colorado River Board found the project feasible , but warned that should the dam fail , every downstream Colorado River community would be destroyed , and that the river might change course and empty into the <unk> Sea . The Board cautioned : " To avoid such possibilities , the proposed dam should be constructed on conservative if not ultra @-@ conservative lines . " 
 On December 21 , 1928 President <unk> signed the bill authorizing the dam . The Boulder Canyon Project Act appropriated $ 165 million for the Hoover Dam along with the downstream Imperial Dam and All @-@ American Canal , a replacement for Beatty 's canal entirely on the U.S. side of the border . It also permitted the compact to go into effect when at least six of the seven states approved it . This occurred on March 6 , 1929 with Utah 's ratification ; Arizona did not approve it until 1944 . 
 
 = = = Design , preparation and contracting = = = 
 
 Even before Congress approved the Boulder Canyon Project , the Bureau of Reclamation was considering what kind of dam should be used . Officials eventually decided on a massive concrete arch @-@ gravity dam , the design of which was overseen by the Bureau 's chief design engineer John L. Savage . The <unk> dam would be thick at the bottom and thin near the top , and would present a convex face towards the water above the dam . The curving arch of the dam would transmit the water 's force into the abutments , in this case the rock walls of the canyon . The wedge @-@ shaped dam would be 660 ft ( 200 m ) thick at the bottom , narrowing to 45 ft ( 14 m ) at the top , leaving room for a highway connecting Nevada and Arizona . 
 On January 10 , 1931 , the Bureau made the bid documents available to interested parties , at five dollars a copy . The government was to provide the materials ; but the contractor was to prepare the site and build the dam . The dam was described in minute detail , covering 100 pages of text and 76 drawings . A $ 2 million bid bond was to accompany each bid ; the winner would have to post a $ 5 million performance bond . The contractor had seven years to build the dam , or penalties would ensue . 
 The <unk> Brothers , heads of the Utah Construction Company , were interested in bidding on the project , but lacked the money for the performance bond . They lacked sufficient resources even in combination with their longtime partners , Morrison @-@ <unk> , which employed the nation 's leading dam builder , Frank Crowe . They formed a joint venture to bid for the project with Pacific Bridge Company of Portland , Oregon ; Henry J. Kaiser & W. A. <unk> Company of San Francisco ; MacDonald & Kahn Ltd. of Los Angeles ; and the <unk> Shea Company of Portland , Oregon . The joint venture was called Six Companies , Inc. as <unk> and Kaiser were considered one company for purposes of 6 in the name . The name was descriptive and was an inside joke among the San Franciscans in the bid , where " Six Companies " was also a Chinese benevolent association in the city . There were three valid bids , and Six Companies ' bid of $ 48 @,@ 890 @,@ <unk> was the lowest , within $ 24 @,@ 000 of the <unk> government estimate of what the dam would cost to build , and five million dollars less than the next @-@ lowest bid . 
 The city of Las Vegas had lobbied hard to be the headquarters for the dam construction , closing its many <unk> when the decision maker , Secretary of the Interior Ray Wilbur came to town . Instead , Wilbur announced in early 1930 that a model city was to be built in the desert near the dam site . This town became known as Boulder City , Nevada . Construction of a rail line joining Las Vegas and the dam site began in September 1930 . 
 
 = = Construction = = 
 
 
 = = = Labor force = = = 
 
 Soon after the dam was authorized , increasing numbers of unemployed people <unk> on southern Nevada . Las Vegas , then a small city of some 5 @,@ 000 , saw between 10 @,@ 000 and 20 @,@ 000 unemployed descend on it . A government camp was established for <unk> and other personnel near the dam site ; this soon became surrounded by a squatters ' camp . Known as <unk> , the camp was home to men hoping for work on the project , together with their families . Another camp , on the flats along the Colorado River , was officially called <unk> , but was known to its inhabitants as " <unk> " . When construction began , Six Companies hired large numbers of workers , with more than 3 @,@ 000 on the <unk> by 1932 and with employment peaking at 5 @,@ 251 in July 1934 . " Mongolian " ( Chinese ) labor was prevented by the construction contract , while the number of blacks employed by Six Companies never exceeded thirty , mostly lowest @-@ pay @-@ scale laborers in a segregated crew , who were issued separate water buckets . 
 As part of the contract , Six Companies , Inc. was to build Boulder City to house the workers . The original timetable called for Boulder City to be built before the dam project began , but President Hoover ordered work on the dam to begin in March 1931 rather than in October . The company built <unk> , attached to the canyon wall , to house 480 single men at what became known as River Camp . Workers with families were left to provide their own accommodations until Boulder City could be completed , and many lived in <unk> . The site of Hoover Dam <unk> extremely hot weather , and the summer of 1931 was especially <unk> , with the daytime high averaging 119 @.@ 9 ° F ( 48 @.@ 8 ° C ) . Sixteen workers and other <unk> residents died of heat <unk> between June 25 and July 26 , 1931 . 
 The Industrial Workers of the World ( <unk> or " <unk> " ) , though much @-@ reduced from their <unk> as militant labor organizers in the early years of the century , hoped to <unk> the Six Companies workers by <unk> on their discontent . They sent eleven organizers , several of whom were arrested by Las Vegas police . On August 7 , 1931 , the company cut wages for all tunnel workers . Although the workers sent away the organizers , not wanting to be associated with the " <unk> " , they formed a committee to represent them with the company . The committee drew up a list of demands that evening and presented them to Crowe the following morning . He was <unk> . The workers hoped that Crowe , the general superintendent of the job , would be sympathetic ; instead he gave a scathing interview to a newspaper , describing the workers as " <unk> " . 
 On the morning of the 9th , Crowe met with the committee and told them that management refused their demands , was stopping all work , and was laying off the entire work force , except for a few office workers and carpenters . The workers were given until 5 p.m. to <unk> the premises . Concerned that a violent confrontation was imminent , most workers took their <unk> and left for Las Vegas to await developments . Two days later , the remainder were talked into leaving by law enforcement . On August 13 , the company began hiring workers again , and two days later , the strike was called off . While the workers received none of their demands , the company guaranteed there would be no further reductions in wages . Living conditions began to improve as the first residents moved into Boulder City in late 1931 . 
 A second labor action took place in July 1935 , as construction on the dam wound down . When a Six Companies manager altered working times to force workers to take lunch on their own time , workers responded with a strike . <unk> by Crowe 's reversal of the lunch decree , workers raised their demands to include a $ 1 @-@ per @-@ day raise . The company agreed to ask the Federal government to supplement the pay , but no money was forthcoming from Washington . The strike ended . 
 
 = = = River diversion = = = 
 
 Before the dam could be built , the Colorado River needed to be diverted away from the construction site . To accomplish this , four diversion tunnels were driven through the canyon walls , two on the Nevada side and two on the Arizona side . These tunnels were 56 ft ( 17 m ) in diameter . Their combined length was nearly 16 @,@ 000 ft , or more than 3 miles ( 5 km ) . The contract required these tunnels to be completed by October 1 , 1933 , with a $ 3 @,@ 000 @-@ per @-@ day fine to be assessed for any delay . To meet the deadline , Six Companies had to complete work by early 1933 , since only in late fall and winter was the water level in the river low enough to safely divert . 
 <unk> began at the lower portals of the Nevada tunnels in May 1931 . Shortly afterward , 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 <unk> , was poured . <unk> cranes , running on rails through the entire length of each tunnel were used to place the concrete . The <unk> were poured next . <unk> sections of steel forms were used for the <unk> . Finally , using <unk> guns , the <unk> were filled in . The concrete lining is 3 feet ( 1 m ) thick , reducing the finished tunnel diameter to 50 ft ( 15 m ) . The river was diverted into the two Arizona tunnels on November 13 , 1932 ; the Nevada tunnels were kept in reserve for high water . This was done by exploding a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble into the river until its natural course was blocked . 
 Following the completion of the dam , the entrances to the two outer diversion tunnels were sealed at the opening and halfway through the tunnels with large concrete <unk> . The downstream halves of the tunnels following the inner <unk> are now the main bodies of the spillway tunnels . The inner diversion tunnels were <unk> at approximately one @-@ third of their length , beyond which they now carry steel pipes connecting the intake towers to the power plant and outlet works . The inner tunnels ' outlets are equipped with gates that can be closed to drain the tunnels for maintenance . 
 
 = = = <unk> , rock clearance and grout curtain = = = 
 
 To protect the construction site from the Colorado River and to facilitate the river 's diversion , two cofferdams were constructed . Work on the upper cofferdam began in September 1932 , even though the river had not yet been diverted . The cofferdams were designed to protect against the possibility of the river flooding a site at which two thousand men might be at work , and their specifications were covered in the bid documents in nearly as much detail as the dam itself . The upper cofferdam was 96 ft ( 29 m ) high , and 750 feet ( 230 m ) thick at its base , as thicker than the dam itself . It contained 650 @,@ 000 cubic yards ( 500 @,@ 000 m3 ) of material . 
 When the cofferdams were in place and the construction site was drained of water , excavation for the dam foundation began . For the dam to rest on solid rock , it was necessary to remove accumulated erosion soils and other loose materials in the riverbed until sound bedrock was reached . Work on the foundation excavations was completed in June 1933 . During this excavation , approximately 1 @,@ 500 @,@ 000 cu yd ( 1 @,@ 100 @,@ 000 m3 ) of material was removed . Since the dam was an arch @-@ gravity type , the side @-@ walls of the canyon would bear the force of the <unk> lake . Therefore , the side @-@ walls were excavated too , to reach virgin rock as weathered rock might provide pathways for water seepage . 
 The men who removed this rock were called " high scalers " . While suspended from the top of the canyon with ropes , the high @-@ scalers climbed down the canyon walls and removed the loose rock with <unk> and dynamite . Falling objects were the most common cause of death on the dam site ; the high scalers ' work thus helped ensure worker safety . One high <unk> was able to save life in a more direct manner : when a government <unk> lost his grip on a safety line and began tumbling down a slope towards almost certain death , a high <unk> was able to intercept him and pull him into the air . The construction site had , even then , become a magnet for tourists ; the high scalers were prime attractions and showed off for the <unk> . The high scalers received considerable media attention , with one worker dubbed the " Human <unk> " for swinging co @-@ workers ( and , at other times , cases of dynamite ) across the canyon . To protect themselves against falling objects , some high scalers took cloth hats and dipped them in tar , allowing them to harden . When workers wearing such headgear were struck hard enough to inflict broken jaws , they sustained no skull damage , Six Companies ordered thousands of what initially were called " hard boiled hats " ( later " hard hats " ) and strongly encouraged their use . 
 The cleared , underlying rock foundation of the dam site was reinforced with grout , called a grout curtain . <unk> were driven into the walls and base of the canyon , as deep as 150 feet ( 46 m ) into the rock , and any cavities encountered were to be filled with grout . This was done to stabilize the rock , to prevent water from seeping past the dam through the canyon rock , and to limit " <unk> " — upward pressure from water seeping under the dam . The workers were under severe time constraints due to the beginning of the concrete pour , and when they encountered hot springs or cavities too large to readily fill , they moved on without resolving the problem . A total of 58 of the 393 holes were incompletely filled . After the dam was completed and the lake began to fill , large numbers of significant leaks into the dam caused the Bureau of Reclamation to look into the situation . It found that the work had been incompletely done , and was based on less than a full understanding of the canyon 's geology . New holes were drilled from inspection galleries inside the dam into the surrounding bedrock . It took nine years ( 1938 – 47 ) under relative secrecy to complete the supplemental grout curtain . 
 
 = = = Concrete = = = 
 
 The first concrete was poured into the dam on June 6 , 1933 , 18 months ahead of schedule . Since concrete <unk> and contracts as it <unk> , the potential for uneven cooling and contraction of the concrete posed a serious problem . Bureau of Reclamation engineers calculated that if the dam was built in a single continuous pour , the concrete would take 125 years to cool and the resulting stresses would cause the dam to crack and crumble . Instead , the ground where the dam was to rise was marked with <unk> , and concrete blocks in columns were poured , some as large as 50 ft square ( 15 m ) and 5 feet ( 1 @.@ 5 m ) high . Each five @-@ foot form contained a series of 1 @-@ inch ( 25 mm ) steel pipes through which first cool river water , then later ice @-@ cold water from a <unk> plant was run . When an individual block had cured and had stopped contracting , the pipes were filled with grout . <unk> was also used to fill the hairline spaces between columns , which were grooved to increase the strength of the joins . 
 The concrete was delivered in huge steel buckets 7 feet high ( 2 @.@ 1 m ) and almost 7 feet in diameter ; Crowe was awarded two patents for their design . These buckets , which weighed 20 short tons ( 18 t ) when full , were filled at two massive concrete plants on the Nevada side , and were delivered to the site in special <unk> . The buckets were then suspended from aerial <unk> , which were used to deliver the bucket to a specific column . As the required grade of aggregate in the concrete differed depending on placement in the dam ( from pea @-@ sized gravel to 9 @-@ inch or 23 cm stones ) , it was vital that the bucket be <unk> to the proper column . When the bottom of the bucket opened up , <unk> 8 cu yd ( 6 @.@ 1 m3 ) of concrete , a team of men worked it throughout the form . Although there are myths that men were caught in the pour and are <unk> in the dam to this day , each bucket only deepened the concrete in a form by an inch , and Six Companies engineers would not have permitted a flaw caused by the presence of a human body . 
 A total of 3 @,@ 250 @,@ 000 cubic yards ( 2 @,@ 480 @,@ 000 m3 ) of concrete was used in the dam before concrete pouring ceased on May 29 , 1935 . In addition , 1 @,@ 110 @,@ 000 cu yd ( 850 @,@ 000 m3 ) were used in the power plant and other works . More than <unk> miles ( <unk> km ) of cooling pipes were placed within the concrete . Overall , there is enough concrete in the dam to pave a two @-@ lane highway from San Francisco to New York . Concrete cores were removed from the dam for testing in 1995 ; they showed that " Hoover Dam 's concrete has continued to slowly gain strength " and the dam is composed of a " durable concrete having a <unk> strength exceeding the range typically found in normal mass concrete " . Hoover Dam concrete is not subject to alkali – <unk> reaction ( <unk> ) as the Hoover Dam builders happened to use <unk> aggregate , unlike that at downstream Parker Dam , where <unk> has caused measurable deterioration . 
 
 = = = Dedication and completion = = = 
 
 With most work finished on the dam itself ( the powerhouse remained uncompleted ) , a formal dedication ceremony was arranged for September 30 , 1935 , to coincide with a western tour being made by President Franklin D. Roosevelt . The morning of the dedication , it was moved forward three hours from 2 p.m. Pacific time to 11 a.m. ; this was done because Secretary of the Interior Harold L. Ickes had reserved a radio slot for the President for 2 p.m. but officials did not realize until the day of the ceremony that the slot was for 2 p.m. Eastern Time . Despite the change in the ceremony time , and temperatures of 102 ° F ( 39 ° C ) , 10 @,@ 000 people were present for the President 's speech in which he avoided mentioning the name of former President Hoover , who was not invited to the ceremony . To mark the occasion , a three @-@ cent stamp was issued by the United States Post Office Department — bearing the name " Boulder Dam " , the official name of the dam between 1933 and 1947 . After the ceremony , Roosevelt made the first visit by any American president to Las Vegas . 
 Most work had been completed by the dedication , and Six Companies negotiated with the government through late 1935 and early 1936 to settle all claims and arrange for the formal transfer of the dam to the Federal Government . The parties came to an agreement and on March 1 , 1936 , Secretary Ickes formally accepted the dam on behalf of the government . Six Companies was not required to complete work on one item , a concrete <unk> for one of the bypass tunnels , as the tunnel had to be used to take in irrigation water until the powerhouse went into operation . 
 
 = = = Construction deaths = = = 
 
 There were 112 deaths associated with the construction of the dam . The first was J. G. <unk> , a surveyor who drowned on December 20 , 1922 , while looking for an ideal spot for the dam . <unk> @-@ six of the deaths occurred during construction at the site . Of the 112 fatalities , 91 were Six Companies employees , three were BOR employees , and one was a visitor to the site , with the remainder employees of various contractors not part of Six Companies . 
 Not included in the official fatalities number were deaths that were recorded as pneumonia . Workers alleged that this diagnosis was a cover for death from carbon monoxide poisoning , brought on by the use of gasoline @-@ fueled vehicles in the diversion tunnels , and a classification used by Six Companies to avoid paying compensation claims . The site 's diversion tunnels frequently reached 140 ° F ( 60 ° C ) , enveloped in thick plumes of vehicle exhaust gases . A total of 42 workers were recorded as having died from pneumonia ; none were listed as having died from carbon monoxide poisoning . No deaths of non @-@ workers from pneumonia were recorded in Boulder City during the construction period . 
 
 = = = Architectural style = = = 
 
 The initial plans for the facade of the dam , the power plant , the outlet tunnels and ornaments clashed with the modern look of an arch dam . The Bureau of Reclamation , more concerned with the dam 's functionality , adorned it with a Gothic @-@ inspired <unk> and eagle statues . 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 Bureau of Reclamation , was brought in to redesign the exteriors . Kaufmann greatly streamlined the design , and applied an elegant Art Deco style to the entire project . He designed <unk> turrets rising seamlessly from the dam face and clock faces on the intake towers set for the time in Nevada and Arizona — the two states are in different time zones , but as Arizona does not observe <unk> Saving Time , the clocks display the same time for more than half the year . 
 At Kaufmann 's request , Denver artist Allen <unk> True was hired to handle the design and decoration of the walls and floors of the new dam . True 's design scheme incorporated motifs of the <unk> and <unk> 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 , textiles , <unk> and ceramics . The images and colors are based on Native American visions of rain , lightning , water , clouds , and local animals — lizards , serpents , birds — and on the Southwestern landscape of stepped <unk> . In these works , which are integrated into the <unk> and interior halls of the dam , True also reflected on the machinery of the operation , making the symbolic patterns appear both ancient and modern . 
 With the agreement of Kaufmann and the engineers , True also devised an innovative color @-@ coding for the pipes and machinery , which was implemented throughout all BOR projects . True 's consulting artist job lasted through 1942 ; it was extended so he could complete design work for the Parker , <unk> and Grand <unk> dams and power plants . True 's work on the Hoover Dam was <unk> referred to in a poem published in The New Yorker , part of which read , " lose the spark , and justify the dream ; but also worthy of remark will be the color scheme " . 
 <unk> Kaufmann and True 's work , the Norwegian @-@ born , naturalized American sculptor <unk> <unk> Hansen designed many of the sculptures on and around the dam . His works include the monument of dedication plaza , a plaque to <unk> the workers killed and the bas @-@ reliefs on the elevator towers . In his words , Hansen wanted his work to express " the immutable calm of intellectual resolution , and the enormous power of trained physical strength , equally enthroned in placid triumph of scientific accomplishment " , because " [ t ] he building of Hoover Dam belongs to the <unk> of the daring . " Hansen 's dedication plaza , on the Nevada <unk> , contains a sculpture of two winged figures flanking a flagpole . 
 Surrounding the base of the monument is a <unk> floor embedded with a " star map " . The map depicts the Northern Hemisphere sky at the moment of President Roosevelt 's dedication of the dam . This is intended to help future astronomers , if necessary , calculate the exact date of dedication . The 30 @-@ foot @-@ high ( 9 @.@ 1 m ) bronze figures , dubbed " <unk> Figures of the Republic " , were each formed in a continuous pour . To put such large bronzes into place without <unk> the highly polished bronze surface , they were placed on ice and guided into position as the ice melted . Hansen 's bas @-@ relief on the Nevada elevator tower depicts the benefits of the dam : flood control , navigation , irrigation , water storage , and power . The bas @-@ relief on the Arizona elevator depicts , in his words , " the <unk> of those Indian tribes who have inhabited mountains and plains from ages distant . " 
 
 = = Operation = = 
 
 
 = = = Power plant and water demands = = = 
 
 Excavation for the powerhouse was carried out simultaneously with the excavation for the dam foundation and abutments . A U @-@ shaped structure located at the downstream toe of the dam , its excavation was completed in late 1933 with the first concrete placed in November 1933 . <unk> of Lake Mead began February 1 , 1935 , even before the last of the concrete was poured that May . The powerhouse was one of the projects uncompleted at the time of the formal dedication on September 30 , 1935 — a crew of 500 men remained to finish it and other structures . To make the powerhouse roof <unk> , it was constructed of layers of concrete , rock , and steel with a total thickness of about 3 @.@ 5 feet ( 1 @.@ 1 m ) , topped with layers of sand and tar . 
 In the latter half of 1936 , water levels in Lake Mead were high enough to permit power generation , and the first three <unk> <unk> built Francis turbine @-@ generators , all on the Nevada side , began operating . In March 1937 , one more Nevada generator went online and the first Arizona generator by August . By September 1939 , four more generators were operating , and the dam 's power plant became the largest <unk> facility in the world . The final generator was not placed in service until 1961 , bringing the maximum generating capacity to 1 @,@ 345 megawatts at the time . Original plans called for 16 large generators , eight on each side of the river , but two smaller generators were installed instead of one large one on the Arizona side for a total of 17 . The smaller generators were used to serve smaller communities at a time when the output of each generator was dedicated to a single municipality , before the dam 's total power output was placed on the grid and made <unk> <unk> . The present contracts for the sale of electricity <unk> in 2017 . 
 Before water from Lake Mead reaches the turbines , it enters the intake towers and then four gradually narrowing <unk> which funnel the water down towards the powerhouse . The <unk> provide a maximum hydraulic head ( water pressure ) of 590 ft ( 180 m ) as the water reaches a speed of about 85 mph ( 140 km / h ) . The entire flow of the Colorado River passes through the turbines . The spillways and outlet works ( jet @-@ flow gates ) are rarely used . The jet @-@ flow gates , located in concrete structures 180 feet ( 55 m ) above the river , and also at the outlets of the inner diversion tunnels at river level , may be used to divert water around the dam in emergency or flood conditions , but have never done so , and in practice are only used to drain water from the <unk> for maintenance . Following an <unk> project from 1986 to 1993 , the total gross power rating for the plant , including two 2 @.@ 4 <unk> <unk> turbine @-@ generators that power Hoover Dam 's own operations is a maximum capacity of <unk> megawatts . The annual generation of Hoover Dam varies . The maximum net generation was 10 @.@ 348 TWh in 1984 , and the minimum since 1940 was 2 @.@ 648 TWh in 1956 . The average power generated was 4 @.@ 2 TWh / year for 1947 @-@ 2008 . In 2015 , the dam generated 3 @.@ 6 TWh . To lower the minimum power pool elevation from 1 @,@ 050 to 950 feet ( 320 to 290 m ) , five wide @-@ head turbines , designed to work efficiently with less flow , will be online by 2017 . Due to lack of water , the dam mostly provides power only during periods of peak demand . 
 Control of water was the primary concern in the building of the dam . Power generation has allowed the dam project to be self @-@ sustaining : proceeds from the sale of power repaid the 50 @-@ year construction loan , and those revenues also finance the <unk> @-@ dollar yearly maintenance budget . Power is generated in step with and only with the release of water in response to downstream water demands . 
 Lake Mead and downstream releases from the dam also provide water for both municipal and irrigation uses . Water released from the Hoover Dam eventually reaches several canals . The Colorado River <unk> and Central Arizona Project branch off Lake <unk> while the All @-@ American Canal is supplied by the Imperial Dam . In total , water from the Lake Mead serves 18 million people in Arizona , Nevada and California and supplies the irrigation of over 1 @,@ 000 @,@ 000 acres ( 400 @,@ 000 ha ) of land . 
 
 = = = = Power distribution = = = = 
 
 Electricity from the dam 's powerhouse was originally sold pursuant to a fifty @-@ year contract , authorized by Congress in 1934 , which ran from 1937 to 1987 . In 1984 , Congress passed a new statute which set power <unk> from the dam from 1987 to 2017 . The powerhouse was run under the original authorization by the Los Angeles Department of Water and Power and Southern California Edison ; in 1987 , the Bureau of Reclamation assumed control . In 2011 , Congress enacted legislation extending the current contracts until <unk> , after setting aside 5 % of Hoover Dam 's power for sale to Native American tribes , electric <unk> , and other entities . The new arrangement will begin in 2017 . The Bureau of Reclamation reports that the energy generated is allocated as follows : 
 
 = = = <unk> = = = 
 
 The dam is protected against over @-@ topping by two spillways . The spillway entrances are located behind each dam <unk> , running roughly parallel to the canyon walls . The spillway entrance arrangement forms a classic side @-@ flow weir with each spillway containing four 100 @-@ foot @-@ long ( 30 m ) and 16 @-@ foot @-@ wide ( 4 @.@ 9 m ) steel @-@ drum gates . Each gate weighs 5 @,@ 000 @,@ 000 pounds ( 2 @,@ 300 @,@ 000 kg ) and can be operated manually or automatically . Gates are raised and lowered depending on water levels in the reservoir and flood conditions . The gates are unable to entirely prevent water from entering the spillways but are able to maintain an extra 16 ft ( 4 @.@ 9 m ) of lake level . Water flowing over the spillways drops dramatically into 600 @-@ foot @-@ long ( 180 m ) , 50 @-@ foot @-@ wide ( 15 m ) spillway tunnels before connecting to the outer diversion tunnels , and <unk> the main river channel below the dam . This complex spillway entrance arrangement combined with the approximate 700 @-@ foot ( 210 m ) elevation drop from the top of the reservoir to the river below was a difficult engineering problem and posed numerous design challenges . Each spillway 's capacity of 200 @,@ 000 cu ft / s ( 5 @,@ 700 m3 / s ) was <unk> verified in post @-@ construction tests in 1941 . 
 The large spillway tunnels have been used only twice , for testing in 1941 and because of flooding in 1983 . During both times , when inspecting the tunnels after the spillways were used , engineers found major damage to the concrete <unk> and underlying rock . The 1941 damage was attributed to a slight <unk> of the tunnel <unk> ( or base ) , which caused <unk> , a phenomenon in fast @-@ flowing liquids in which vapor bubbles collapse with explosive force . In response to this finding , the tunnels were patched with special heavy @-@ duty concrete and the surface of the concrete was polished mirror @-@ smooth . The spillways were modified in 1947 by adding flip buckets , which both slow the water and decrease the spillway 's effective capacity , in an attempt to eliminate conditions thought to have contributed to the 1941 damage . The 1983 damage , also due to <unk> , led to the installation of <unk> in the spillways . Tests at Grand <unk> Dam showed that the technique worked , in principle . 
 
 = = = <unk> and tourism = = = 
 
 There are two lanes for automobile traffic across the top of the dam , which formerly served as the Colorado River crossing for U.S. Route 93 . In the wake of the September 11 , 2001 terrorist attacks , authorities expressed security concerns and the Hoover Dam Bypass project was expedited . <unk> the completion of the bypass , restricted traffic was permitted over Hoover Dam . Some types of vehicles were inspected prior to crossing the dam while semi @-@ trailer trucks , buses carrying luggage , and enclosed @-@ box trucks over 40 ft ( 12 m ) long were not allowed on the dam at all , and were diverted to U.S. Route 95 or Nevada State Routes 163 / 68 . The four @-@ lane Hoover Dam Bypass opened on October 19 , 2010 . It includes a composite steel and concrete arch bridge , the Mike O <unk> – Pat <unk> Memorial Bridge , 1 @,@ 500 ft ( 460 m ) downstream from the dam . With the opening of the bypass , through traffic is no longer allowed across Hoover Dam , dam visitors are allowed to use the existing roadway to approach from the Nevada side and cross to parking lots and other facilities on the Arizona side . 
 Hoover Dam opened for tours in 1937 after its completion , but following Japan 's attack on Pearl Harbor on December 7 , 1941 , it was closed to the public when the United States entered World War II , during which only authorized traffic , in convoys , was permitted . After the war , it reopened September 2 , 1945 , and by 1953 , annual attendance had risen to 448 @,@ 081 . The dam closed on November 25 , 1963 and March 31 , 1969 , days of mourning in <unk> of Presidents Kennedy and Eisenhower . In 1995 , a new visitors ' center was built , and the following year , visits exceeded one million for the first time . The dam closed again to the public on September 11 , 2001 ; modified tours were resumed in December and a new " Discovery Tour " was added the following year . Today , nearly a million people per year take the tours of the dam offered by the Bureau of Reclamation . Increased security concerns by the government have led to most of the interior structure being inaccessible to tourists . As a result , few of True 's decorations can now be seen by visitors . 
 
 = = Environmental impact = = 
 
 The changes in water flow and use caused by Hoover Dam 's construction and operation have had a large impact on the Colorado River Delta . The construction of the dam has been credited as causing the decline of this estuarine ecosystem . For six years after the construction of the dam , while Lake Mead filled , virtually no water reached the mouth of the river . The delta 's estuary , which once had a freshwater @-@ <unk> mixing zone stretching 40 miles ( 64 km ) south of the river 's mouth , was turned into an <unk> estuary where the level of salinity was higher close 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 threatened many species adapted to the flooding , including both plants and animals . The construction of the dam devastated the populations of native fish in the river downstream from the dam . Four species of fish native to the Colorado River , the <unk> <unk> , Colorado <unk> , <unk> <unk> , and Razorback <unk> , are listed as endangered . 
 
 = = Naming controversy = = 
 
 During the years of lobbying leading up to the passage of legislation authorizing the dam in 1928 , the press generally referred to the dam as " Boulder Dam " or as " Boulder Canyon Dam " , even though the proposed site had shifted to Black Canyon . The Boulder Canyon Project Act of 1928 ( <unk> ) never mentioned a proposed name or title for the dam . The <unk> 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 " . 
 When Secretary Wilbur spoke at the ceremony starting the building of the railway between Las Vegas and the dam site on September 17 , 1930 , he named the dam " Hoover Dam " , citing a tradition of naming dams after Presidents , though none had been so honored during their terms of office . Wilbur justified his choice on the ground that Hoover was " the great engineer whose vision and persistence ... has done so much to make [ the dam ] possible " . One writer complained in response that " the Great Engineer had quickly drained , <unk> , and <unk> the country . " 
 After Hoover 's election defeat in 1932 and the accession of the Roosevelt administration , Secretary Ickes ordered on May 13 , 1933 that the dam be referred to as " Boulder Dam " . Ickes stated that Wilbur had been <unk> in naming the dam after a sitting president , that Congress had never ratified his choice , and that it had long been referred to as Boulder Dam . Unknown to the general public , Attorney General Homer Cummings informed Ickes that Congress had indeed used the name " Hoover Dam " in five different bills appropriating money for construction of the dam . The official status this conferred to the name " Hoover Dam " had been noted on the floor of the House of Representatives by Congressman Edward T. Taylor of Colorado on December 12 , 1930 , but was likewise ignored by Ickes . 
 When Ickes spoke at the dedication ceremony on September 30 , 1935 , he was determined , as he recorded in his diary , " to try to nail down for good and all the name Boulder Dam . " At one point in the speech , he spoke the words " Boulder Dam " five times within thirty seconds . Further , he suggested that if the dam were to be named after any one person , it should be for California Senator <unk> Johnson , a lead sponsor of the authorizing legislation . Roosevelt also referred to the dam as Boulder Dam , and the Republican @-@ leaning Los Angeles Times , which at the time of Ickes ' name change had run an editorial cartoon showing Ickes <unk> <unk> away at an enormous sign " <unk> <unk> , " <unk> it showing Roosevelt reinforcing Ickes , but having no greater success . 
 In the following years , the name " Boulder Dam " failed to fully take hold , with many Americans using both names <unk> and <unk> divided as to which name should be printed . Memories of the Great Depression faded , and Hoover to some extent rehabilitated himself through good works during and after World War II . In 1947 , a bill passed both Houses of Congress unanimously restoring the name " Hoover Dam . " Ickes , who was by then a private citizen , opposed the change , stating , " I didn 't know Hoover was that small a man to take credit for something he had nothing to do with . " 
 
 