Monitoring Channel Sand Storage in the Colorado River in Grand Canyon

Sandbars provide habitat for plants and wildlife along the Colorado River in the Grand Canyon and are commonly the only flat areas among cliffs and boulders where river rafters can camp. As part of the Glen Canyon Environmental Studies, funded and coordinated by the Bureau of Reclamation, the U.S. Geological Survey is monitoring the supply, storage, and movement of sand in Grand Canyon National Park. Before Glen Canyon Dam was built, an average of about 65 million tons of sediment were transported by the Colorado River into the park each year. Because the dam now blocks the river, the only significant sources of sediment for the park are streams and rivers downstream from the dam. The amount of sand these tributaries supply varies greatly from year to year but averages about 2.9 million tons per year. Available data indicate that if dam releases are managed properly this is enough sand to maintain sandbars and other sand deposits in the park (Bureau of Reclamation, 1994, p. 50-51). Sand from tributaries is temporarily deposited in the river channel during low flows produced by controlled dam releases. A controlled flood of appropriate size and duration will scour sand and rebuild some sandbars. The monitoring study was designed to measure the changes in channel sand storage caused by tributary floods and the low releases from the dam in 1992 and 1993 (with daily maximum releases restricted to less than about 20,000 cubic feet per second). The data collected also are being used to refine computer models that can simulate the movement of sand through reaches of the river.