Waterfalls In Reservoirs: Tracking The Development of Nickpoints In Reservoir Sediments As Drought Drains Lakes Powell and Mead

Recent drought has resulted in dramatic lowering of the two largest water-storage reservoirs in the western United States: Lake Powell and Lake Mead. These decreases in reservoir elevations have resulted in the re-emergence of over 100 km of the Colorado and San Juan rivers at the upstream end of Lake Powell and over 80 km of the Colorado River at the upstream end of Lake Mead. Upon reservoir lowering, the rivers cut into the lake and delta deposits sometimes establishing a course different than the path of the historical channel. In two cases, the rivers have encountered resistant bedrock resulting in the formation of knickpoints and rapids or waterfalls. In a third case, a knickpoint is just beginning to form and a new rapid or waterfall appears imminent if water levels continue to drop. These reservoir-waterfalls have important implications for river and reservoir management. As knickpoints, they control the upstream energy gradient and affect the rate and pattern of evacuation of exposed reservoir delta sediments, thus affecting river dynamics and river ecosystems far upstream. They also have immediate impacts in the vicinity of the knickpoint on navigation, which affects decisions regarding expensive infrastructure such as boat ramps. They also create ecological barriers that impact the migration of native and nonnative fish. We present descriptions of these phenomena, using repeat measurements with multibeam sonar of the riverbed to document scour at the site of nickpoint formation.