STRATIGRAPHIC EVIDENCE FOR CLIMATE-DRIVEN IMBALANCES IN THE FINE SEDIMENT BUDGET OF THE COLORADO RIVER DURING THE PLEISTOCENE

Fine-grained deposits along the lower Colorado River record at least one episode of major river aggradation followed by downcutting during the Pleistocene. The deposits – named the Chemehuevi Formation by Longwell in 1936 – are discontinuous, occupy protected areas along the valley up to 100 m above the historic (i.e., pre-dam) river, and contrast starkly with older gravel deposits along the river corridor. The Chemehuevi remnants have been variously interpreted as resulting from lacustrine conditions, catastrophic floods, or unspecified climate change in the Colorado River watershed. Recent field evidence suggests that the deposits are remnants of one or more temporary aggradational episodes that occurred following changes in the supply of sediment and water from upstream, due to Pleistocene climate changes. A simple model to interpret the deposits is that fine-grained, Chemehuevi-type aggradation events are caused by increases in the ratio of fine sediment to water (SWR) delivered from the watershed. In contrast, reductions in the SWR lead to river downcutting (and coarsening of the channel substrate). Thus, a climatic shift such as the onset of semiarid, monsoonal conditions on the Colorado Plateau and/or the Great Basin – which increase the sediment supply and decrease the water supply – should lead to conditions favoring Chemehuevi aggradation events. Alternatively, a climatic shift such as increased snowmelt in the Rocky Mountains should cause a reduction in the SWR and instigate downcutting in the lower Colorado River. Documenting the origin, stratigraphy, composition, and age(s) of the Chemehuevi remnants will help improve the reconstructions of past climate change and predict the response of the river to fluctuations in the sediment budget. Field observations, sample analyses, and secondary data were used to characterize aspects of the stratigraphy, distribution, age and composition of many of the Chemehuevi remnants between Grand Canyon and the US-Mexico border. Preliminary calculations of the volume of the deposit based on field surveys are compared with estimates of the fine-sediment load of the lower Colorado River. The results of this study are meant to inform predictive models about the evolution of the Colorado River valley alluvial fill in response to changing boundary conditions during Pleistocene, Holocene, and historic times.