They partly bury several mountain ranges and conceal the Imperial Fault, a branch of the famous San Andreas Fault zone. Flow is regulated by the Glen Canyon Dam, built in 1964 upstream from the canyon. Below the canyon are other dams, notably Hoover Dam built in the 1930s, that provide water for power generation and irrigation. Lake Mead, behind Hoover Dam, acts as a settling basin for the river's sediment, so that most of the sediment no longer reaches the delta. But these changes are so recent as to have little meaning in terms of geologic history.
Sand is loose gritty material made up of mineral or rock grains.
Grains range in size from those you can just barely see to an upper limit of one-tenth inch. When we use the term sand by itself we generally mean quartz sand. There is much more quartz sand than any other kind. Quartz is crystalline silica, SiO 2 . When quartz-bearing rocks are weathered and eroded, less resistant minerals are worn away or altered to different minerals, but quartz is unaffected.