Dynamics and Evolution of Tributary Alluvial Fans in the Grand Canyon below Glen Canyon Dam

Alex Morelan

Abstract: Tributary alluvial fans along the Grand Canyon are key to both natural and boater habitat. The primary influence that tributary fans have on the river is as source of sediment input and constriction of main stem flow. The sediment is delivered largely as debris flows: a mix of water, mud, and boulders that often behaves as a non-Newtonian fluid. Because of the unique mechanical attributes of debris flows, they can transport large boulders long distances. This ability to transport boulders becomes important for understanding how tributary alluvial fans influence habitat along the Colorado River at the bottom of the Grand Canyon. As tributary alluvial fans form in the canyon bottom, they constrict the flow of the river forming rapids at the toes of the fans and eddies above and below the constriction (figure 1) (Yanites et al., 2006). Boulders sourced from the fan toe contribute to turbulence in rapids (Hanks and Webb, 2006). Since the completion of Glen Canyon Dam, the dynamics of sediment transport have been significantly interrupted. Large annual variation in flow rate (maximum annual flows up to ~100,000 ft3/s, with an historical record above 300,000 ft3/s) of the Colorado River has been replaced by daily fluctuations (maximum up to ~10,000 ft3/s) as a function of energy prices (Dolan et al., 1974). In the last 20 years annual (1-week duration) high flow events (HFEs) have attempted to mimic pre-dam dynamics (maximum ~50,000 ft3/s) (Alvarez and Schmeeckle, 2013). These HFEs are called upon to serve many functions (including building high sand bars) but have not necessarily been successful in maintaining anything resembling a natural (pre-dam) system. The replacement of historic annual variation with modern high frequency, low-amplitude variation has significantly changed how tributary alluvial fans deliver sediment and interact with the main channel of the Grand Canyon.