USGS scientists have long been involved in studying how dams change the characteristics of river channels downstream from them and they have made major advances in understanding the processes responsible for building and eroding sand bars, the long-term storage and transport of sand in the river, and the supply of new sediment from debris flows and tributary systems. An example is the Grand Canyon National Park study in which USGS, the US Bureau of Reclamation, the National Park Service, US Fish and Wildlife Service, and the Arizona Game and Fish Department cooperated to carry out the first managed flood for environmental purposes. After the closure of Glen Canyon Dam in 1963, the supply of sediment to the Colorado River was cut off, resulting in a loss of sand bars in Grand Canyon National Park. USGS computer models predicted that a large flood would have sufficient energy to suspend the sand in the river channel and deposit bars higher up on the river margins. This led to seven days of a controlled flood that began on March 26, 1996 with the release of 45,000 cubic feet of water. The photographs here show deposits at mile 122 in the Grand Canyon at: 1) preflood conditions, with an open sand bar; 2) the experimental flood with the sand bar under water; and 3) postflood conditions with a larger and thicker sandbar, although some of the new sand bar has since eroded.

Prior to this experiment, there was concern that the downstream plant community might be destroyed by the flood. Although the flood scoured some of the plant biomass from the river channel below the dam, a significant amount was left as evidenced by the the fact that the daily variation in oxygen concentration and pH was measurable immediately after the flood. The plant community was able to rapidly recover. For additional information, see the fact sheet, Effects of Flooding on Plant Production Downstream from Glen Canyon Dam, or contact: