<Tr> <Th> Added to NRHP </Th> <Td> April 8, 1981 </Td> </Tr> <Tr> <Th> Designated NHL </Th> <Td> August 20, 1985 </Td> </Tr> <P> Hoover 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 . Originally known as Boulder Dam from 1933, it was officially renamed Hoover Dam by a joint resolution of Congress in 1947 . The dam was named after President Herbert Hoover . </P> <P> 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 torrid 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 . </P>

When was the hoover dam built and how long did it take
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