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 Colorado River basin. Below the dam the river passes under the Mike O'Callaghan–Pat Tillman Memorial Bridge – which at nearly above the water is the highest concrete arch bridge in the Western Hemisphere – and then turns due south towards Mexico, defining the Arizona–Nevada and Arizona–California borders. After leaving the confines of the Black Canyon, the river emerges from the Colorado Plateau into the Lower Colorado River Valley (LCRV), a desert region dependent on irrigation agriculture and tourism and also home to several major Indian reservations. The river widens here to a broad, moderately deep waterway averaging wide and reaching