A FISTFUL OF SAND: MONITORING THE FATE OF FINE-GRAIN SEDIMENT IN THE COLORADO RIVER, GRAND CANYON, ARIZONA

The 1992 Grand Canyon Protection Act established the Grand Canyon Monitoring and Research Center (GCMRC) to monitor the effects of Glen Canyon Dam on the Colorado River ecosystem in Glen Canyon National Recreation Area and Grand Canyon National Park. As part of this effort, a cooperative partnership, informally named the Fine-grained Integrated Sediment Team (FIST), was formed between the GCMRC, U.S. Geological Survey Research and Coastal and Marine Geology Programs, Utah State University, and Northern Arizona University to investigate the effects of dam operations on the spatial distribution, morphology, and grain-size of fine-grained sediment deposits in the Colorado River in Grand Canyon. Ultimately, results from the FIST will be used to alter dam operations in order to sustain the fine-grained sediment resources along the Colorado River corridor in Grand Canyon National Park.

We focused our efforts on 11, 2-6km reaches of the river corridor and use a GIS-based approach in analyzing changes in the channel bed. Within each reach, we develop detailed DTM’s of the topography and sampled the grain-size along the bed and banks of the river, then repeat these measurements for change detection analysis. The reach DTM’s are constructed by combining multibeam bathymetry of the channel with photogrammetrically derived contours and topographic data points collected from total stations. Overlays of grain-size and substrate type are then constructed. Volumes and areas are calculated within specific geomorphic environments to determine the relative importance of these environments in the sediment budget of the reach. To understand these reach-scale processes, we also compared our observations of fine sediment volume, grain size, and distribution with data on tributary sediment supply and mainstem transport collected by other GCMRC research programs.

These data are collected biannually or before and after experimental dam releases. We have successfully acquired data in August and September 2000, and in May 2002. Results from these surveys will be presented along with the challenges of collecting and integrating large datasets in a difficult environment.