Watch LookMovie. The Hoover Dam is a massive structure situated on the Colorado River, spanning the border of Arizona and Nevada in the United States. It stands 722 feet tall, 660 feet wide at its base, and 1,244 feet long, making it one of the largest dams in the world. Below are Key Construction Timeline Dates: - The Hoover Dam's initial early infrastructure work began in July 1930. - The official start date was April 20, 1931. - The first concrete placement in the dam structure itself was June 6, 1933. - The Dam was dedicated by FDR on September 30, 1935. - The official completion date was March 1st 1936.