Risks to the Glen Canyon Dam that the resulting flood would scour the bottom few hundred feet of the Grand Canyon, overtop Hoover Dam, and cause severe damage all along the lower Colorado River. In the event of a complete dam failure with Lake Powell at high pool, about of water would surge out of Lake Powell at an initial depth of over . Downstream communities and perhaps every dam along the river, excluding possibly Hoover Dam, as well as riverside lowlands, would be inundated or severely damaged. Located downstream from Glen Canyon Dam are Hoover Dam forming Lake Mead, Davis Dam forming Lake Mohave, Parker
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,