History of the Grand Canyon area stretched along a 278-mile (447 km) segment of the Colorado River from the southern border of Glen Canyon National Recreation Area to the eastern boundary of Lake Mead National Recreation Area. Grand Canyon National Park was designated a World Heritage Site on October 24, 1979. In 1935, Hoover Dam started to impound Lake Mead south of the canyon. Conservationists lost a battle to save upstream Glen Canyon from becoming a reservoir. The Glen Canyon Dam was completed in 1966 to control flooding and to provide water and hydroelectric power. Seasonal variations of high flow and flooding in the spring and
Colorado River affected by upstream diversions and reservoir evaporation, especially after the completion of the Colorado River Storage Project in the 1970s. Prior to the completion of Glen Canyon Dam in 1964, the average discharge recorded between 1912 and 1962 was , per year. The drainage basin or watershed of the Colorado River encompasses of southwestern North America, making it the seventh largest on the continent. About , or 97 percent of the watershed, is in the United States. The river and its tributaries drain most of western Colorado and New Mexico, southwestern Wyoming, eastern and southern Utah, southeastern Nevada and California,