PATTERNS OF RIVERBED SAND-STORAGE CHANGE ON THE COLORADO RIVER IN GRAND CANYON

Decades of research on alluvial sandbars and sand transport on the Colorado River in Grand Canyon have contributed to the implementation of management actions, such as curtailed dam operations and controlled floods, intended to rebuild eroded sandbars. This research has also contributed to advances in our understanding of morphological changes in river channels in the context of high-resolution sediment budgets. In a closed sediment budget, measurements of sediment influx and efflux are coupled with measured changes in channel topography to provide independent estimates of both spatial and temporal resolution of changes in the sediment mass balance. For sediment budgets constructed over long river segments (>10² channel widths) and long periods (~2 years or longer), spatial and temporal accumulation of measurement uncertainty, compounded by inadequate sampling frequency or density, may produce indeterminate results. The degree of indeterminacy may be evaluated in the context of a signal-to-noise ratio (SNR), which is a function of the magnitude of the mass balance and the magnitude of uncertainties.

We report on a closed sand budget consisting of measurements of flux and two morphological surveys for a 50-km segment of a large river over a 3-year period. Accurate reporting of the magnitude and sign of the change in sand storage was only possible by using of state-of-the-art suspended-sediment measurement techniques with high temporal and spatial resolutions. In concert with these measurements, a sand-flux and morphological mass balance revealed that sand evacuation was temporally concentrated (~100% of mass change occurred during 19% of the study period) and highly localized (70% of mass change occurred in 12% of the study segment). An SNR analysis revealed that uncertainty resulting from undersampling may exceed that caused by measurement uncertainty and that daily sampling of suspended-sand concentration or repeat mapping of at least 60% of the river segment was required to determine the sand budget with SNR > 1.