GEOMORPHIC AND SEDIMENTOLOGIC EXPRESSIONS OF EXPOSED LAKE POWELL SEDIMENT IN THE RESERVOIR-AFFECTED ZONES OF THE COLORADO RIVER AND SAN JUAN RIVER CORRIDORS

Construction of the Glen Canyon Dam in 1963 created Lake Powell, the second largest water reservoir in the U.S. The Colorado and San Juan rivers and other tributaries were impounded by the reservoir, resulting in the deposition of vast quantities of sediment over a ~650 km² area. Much of this deposition occurs as deltas at the dynamic river-reservoir transition. Under regional drought, increased water consumption, and falling reservoir levels, these sediments have been well exposed for the first time in several decades. This ephemeral sediment archive presents opportunities to research sediment transport, deposition, and erosion through this landscape. In addition, the sedimentary record is contextualized by a comprehensive instrument record encompassing virtually the full 60+ year reservoir history. The rapidly moving sediment is increasingly being recognized as a significant challenge by management agencies: it impacts habitat for a variety of species, affects recreation and navigation, reduces overall water storage volume in Lake Powell, and its remobilization as reservoir levels fluctuate may impact hydropower production at Glen Canyon Dam. Here we synthesize field observations of exposed sediment from 2019-2023 along the reservoir-impacted zone of the Colorado and San Juan rivers. These observations are integrated with remote sensing analyses of landscape change, ongoing environmental monitoring, and analysis of historical imagery for comparison with current conditions. Results document a complex interplay between depositional and diagenetic processes in these fluvial-deltaic-lacustrine deposits and staggering rates of change throughout the system. While similar in some ways, our observations of reservoir sediment in the Colorado and San Juan rivers also highlight distinctions between the two systems. In addition to their societal relevance, these sediments provide an inadvertent landscape scale flume experiment and hold important implications for interpreting analogous systems in the rock record.