MAPPING THE COLORADO RIVER IN GRAND CANYON TO DIRECTLY MEASURE LONG-TERM CHANGES IN SEDIMENT STORAGE

High-resolution (1-m grid) Digital Elevation Models of the river corridor have been constructed for 162 miles of the Colorado River in Glen, Marble and Grand Canyon to directly measure changes in sediment (primarily sand) storage over the scale of long-term operations of Glen Canyon Dam. Changes in sediment storage are evaluated in the context of a mass-balance sediment budget within individual reaches of the river corridor by differencing DEMs at 3-5 year intervals. Monitoring reaches are defined by the location of suspended sediment monitoring stations. Within each reach, sediment mass balance is estimated from both suspended sediment measurements (flux-based budget) and differencing of DEMs (morphologic-based mass balance). The surveys have been collected over a nine-year period (2009-2017), where each survey expedition mapped the channel between sediment monitoring gages. To date, the following long reaches have been mapped: 1) river miles -15 to 0 (2014); 2) 0-32 (in 2013 and 2016); 3) 30-61 (in 2009 and 2012); 4) 60-86 (in 2011 and 2014); and 5) 165 to 225 (2017). To date, 142 km of the 260 km mapped river have been repeated.

The DEMs were constructed by combining data collected using three survey methods: 1) multibeam echosounder surveys; 2) singlebeam echosounder surveys; and 3) ground-based total station surveys. Data from all three survey methods are spatially positioned by occupying benchmarks within a geodetic control network that provides local accuracies of 3 cm, both horizontally and vertically. Positions of sonar systems are telemetered at 20 Hz to vessels using robotic range-azimuth systems situated over benchmarks. Edited survey data points are used to construct Triangulated Irregular Network (TIN) terrain models of the reach, and then the TIN models are converted to 1-m resolution DEMs for analysis. DEM uncertainty is estimated by both fuzzy inference system (FIS) modeling and computing the absolute difference between features that are assumed not to have changed between surveys (so-called fiducial surfaces, such as the tops of rocks). The FIS approach models spatially-distributed uncertainty, incorporating information about surface roughness and slope. The fiducial surface approach results in a scalar uncertainty applied to the entire DEM, and is computed for each of the three survey methods.