845,000 Number of dams in the world. The United States has 80,000, with a total storage capacity of 48 trillion cubic feet of water. Hoover Dam, straddling the Nevada-Arizona border at Lake Mead, is the country’s largest, storing 1.2 trillion cubic feet.
49 Number of dam failures in the United States between 2000 and 2007. Overtopping due to poor design accounts for 34 percent of all failures. Some 85 percent of all large dams will have passed their projected life spans by 2020. The Association of State Dam Safety Officials estimates that it would cost $10.1 billion to repair the dams most in need of rehabilitation.
$25 billion Projected final cost of the Three Gorges Dam in China. Construction of this dam—the world’s largest, holding back 1.4 trillion cubic feet of water—has displaced at least 1.3 million people. Thirteen cities, 140 towns, and 1,350 villages have been intentionally flooded. When complete, the 410-mile-long reservoir will generate 84.7 billion kilowatt-hours of electricity per year, the energy equivalent of 50 million tons of coal.
65 Percentage of ocean-bound freshwater flow that is obstructed by dams. More than half of the world’s 292 large river systems are adversely affected by dams. Water development, including damming, harms 91 percent of endangered fish species and 22 percent of endangered birds in the United States, according to a study published in Bioscience [subscription required].
250 billion Kilowatt-hours of U.S. hydropower production in 2007, representing 6 percent of the country’s total electricity supply. Washington, Oregon, and California are the top hydroelectric producers in the nation. Worldwide, 19 percent of electricity comes from hydropower. The top-generating countries are Canada, the United States, and Brazil.