A Tale of Two Deltas: The Link Between Lake Level Change and Sedimentation in Lake Powell as Recorded by Sediment Cores from the San Juan River Delta and the Colorado River Delta

Lake Powell, a major reservoir in southern Utah, offers a unique opportunity to investigate some of the basic principles of sedimentary geology. It was dammed in 1963, and environmental conditions (e.g., daily lake level, river discharge, precipitation, upstream events) have been meticulously recorded ever since. Deltas have formed at the two major river inputs, the San Juan River (SJR) and the Colorado River (COR). Sediment cores that encompass the entire deltaic package provide insight into sedimentation from 1963 to 2018. Cores in the SJR and COR were characterized by grain size, color, sedimentary structures, and geochemistry. Lake level history and river discharge and turbidity were then compared to the cores to understand how external forcings relate to sediment packages. For the SJR, the lithology is primarily fine-grained mud, with starkly colored red, bleached, and black intervals among brown intervals, suggesting variable redox conditions. For the COR, the lithology is on the whole coarser-grained, and has a more consistent brown coloration. Between the two, the COR is a more characteristic delta, with sedimentation clearly linked to lake level change over the years. This is largely due to the fact that the Colorado River contributes the majority of water input into Lake Powell, and thus the COR is more sensitive to local base level. In the SJR, sedimentation is not so clearly linked to lake level change, and geochemical signals and color variations suggest it is dominated by cyclic, event-driven sedimentation perhaps reflecting variations in sediment supply rather than river discharge. Despite being subject to the same lake level change, the two deltas are functionally independent and show markedly different styles of sedimentation. These two deltas show how the relationship between base level changes and sedimentation can vary even under similar formative conditions, and thus may inform sequence stratigraphic interpretations of the rock record.