Sandbar deposition caused by high-flow experiments on the Colorado River downstream from Glen Canyon Dam: November 2012 – November 2018

The streamflow regime and sand supply of the Colorado River have been affected by the presence and operations of Glen Canyon Dam since filling of Lake Powell began in March 1963. Consequent changes in river morphology have included decreases in the size and abundance of sandbars used as campsites in Grand Canyon National Park (Dolan and others, 1974; Schmidt and Graf, 1990; Kearsley and others, 1994). The sandbars that occur along the banks of the Colorado River and create camping beaches and backwaters and are habitat used by native fish (Dodrill and others, 2015) are inherently unstable features of an active river channel (Schmidt, 1990). The deposits form when sand, carried in suspension in the main channel of the Colorado River, settles in the lower velocity recirculating currents of eddies (Rubin and others, 1990; Schmidt, 1990). When the flows that resulted in bar deposition recede, leaving a fresh sand deposit, the deposits begin eroding. Decreases in the magnitude and frequency of annual floods have resulted in decreased opportunities for deposition. The complete elimination of sand sources upstream from Glen Canyon Dam decrease the supply that is available when high flows do occur. Finally, the increase in the magnitude of flows throughout the year coupled with daily fluctuations for hydropower generation accelerate rates of erosion for remaining sandbars (Hazel and others, 2010).