Flow Alteration, River Valley Morphology, and the Influence of Glen Canyon Dam on Sediment Availability along the Colorado River in Grand Canyon

Through alterations to streamflow, sediment supply, and riparian land cover, dams have the potential to drive wholesale alterations in the availability and movement of sediment within river valleys, with implications for the eco-geomorphic functioning and connectivity of landscape processes. Although the localized effects of dam construction, operation, and removal on river valleys have been studied at the reach scale, the influence of dams on fluvial processes at the larger segment (e.g. 10-100 km) and basin scales is not well characterized. Within river segments and entire basins, river response to dams may vary widely as a function of local channel and valley morphology. Here we use a 95-year legacy of in-situ and remote sensing data on streamflow and riparian land cover to quantify the impacts of Glen Canyon Dam, a large hydropower and water storage facility constructed in 1963, on downstream sediment availability along a 169 kilometer segment of the Colorado River within Grand Canyon National Park, Arizona, USA. Our results indicate the influence of large-scale flow regime shifts, coupled with the role of river valley morphology, on sediment coverage throughout the river corridor. During the recent period 2008-2016, 72% of all sand not covered by riparian vegetation within the study segment was inundated continuously by the river; this value varied from 51% in a wide, alluvial river sub-reach, to 83% in a narrow, canyon-bound sub-reach. When comparing the pre- and post-dam periods, we find that pre-dam median flows would have exposed, on average, 24% more unvegetated sand annually than at present, due in large part to elevated base flows in the post-dam period. When sand is not persistently inundated by river flows, it is available for colonization by many riparian plant species and for sediment transport by fluvial, hillslope, and aeolian geomorphic processes. Through the use of large-scale datasets on biophysical river valley characteristics, our results provide a framework for continuously interpreting the effects of dam construction and operation at large spatial scales, and may provide context in support of sustainable dam management and river restoration.