Geomorphic response of sandbars to the March 2008 high-flow experiment on the Colorado River downstream from Glen Canyon Dam

The completion of Glen Canyon Dam in 1963 drastically altered the downstream flow regime and resulted in more than a 90 percent reduction of sand supply to the Colorado River in Grand Canyon National Park. Sandbars that were maintained by annual floods and a large sediment supply are now fewer in number and smaller in area and volume. Efforts to maintain sandbars in the current era of dam management utilize controlled floods timed to occur during brief periods of sediment enrichment that result from tributary floods. Repeat surveys of 22 sandbars made before and after controlled floods conducted in 1996, 2004, and 2008 document changes in sandbar volume; and repeat surveys at more than 100 sites document changes in sandbar elevation and morphology for the 2008 event. Each of the controlled floods resulted in sandbar deposition that was followed by erosion in the 6-month post-flood period. Erosion rates are positively correlated with post-flood dam release volumes and negatively correlated with post-flood tributary sediment supply volume. October 2008 sandbar volume was similar or larger than sandbar volume in February 1996, before the first of the three controlled floods. Deposition during the 2008 controlled flood was also associated with increases in the quantity of backwater habitat, which is used by native and non-native fish.