Topographic and bathymetric changes at thirty-three long-term study sites

This study documents the geomorphic response to the 1996 controlled flood of 33 study sites in fan-eddy complexes distributed throughout Grand Canyon. Repeated topographic mapping was used to quantify sediment redistribution at sites within three distinct parts of the fan-eddy complex: high-elevation sand bars, submerged areas of the recirculation zone (eddy), and the adjacent main channel. The topographic data show that the 1996 controlled flood rebuilt previously eroded high-elevation bars, regardless of location, bar type, or width of the canyon. The average thickness of new, high-elevation deposits was 0.64 m. Because there was an average thickness decrease in the main channel of 0.45 m, we conclude that large volumes of sediment were scoured from storage locations on the bed. Although there was variability between the behaviors of individual sites, two response styles were identified: (1) deposition filled or nearly filled all areas of small recirculation zones, and (2) extensive low-elevation scour caused net erosion in large recirculation zones. The average recirculation zone change at all sites was net aggradation. Aggradation was greatest in narrow reaches where stage change was greatest. The magnitude of deposition was greatest in the Marble Canyon reach between the Paria River and the Little Colorado River. Sites located downstream from the Little Colorado River still remained partly filled with sediment by the 1993 Little Colorado River floods so the potential for bar filling was limited compared to the sites located upstream. Our results suggest that the amount of high-elevation deposition is in part controlled by the space available for deposition in the recirculation zone before the flood, which we term “accommodation space.” Erosion rates of newly-deposited sand were initially high following the 1996 controlled flood but decreased with time. Our results show that controlled flooding restores eroded bars downstream from Glen Canyon Dam, and produces sand volume increases in bar storage that are at least partially maintained for nearly a year afterward.