The impact of bedrock meander cutoffs on 50 kyr scale incision rates, San Juan River, Utah

Abstract

Incision rates of major rivers may reflect the effects of drainage reorganization, hillslope processes, tectonic uplift, climate, the properties of rocks into which rivers incise, and other autogenic processes. On the Colorado Plateau, incision rates along the Colorado River have been interpreted as resulting from abrupt base-level changes produced by the integration of the Colorado River system. Specifically, the integration of the Colorado River in the location of Grand Canyon is thought to have created a knickpoint, enhanced by lithologic contrasts, which is propagating upstream. While evidence exists for a < 1 Ma acceleration of incision on parts of the Colorado River, uncertainty about the processes reflected in shorter-term incision rates muddies comparison with longer-term averages. In this work, we combine a cosmogenic radionuclide depth profile exposure age and post-infrared infrared stimulated luminescence (p-IR IRSL) to date fluvial deposits adjacent to the San Juan River, a major tributary of the Colorado River, near Mexican Hat, Utah. The deposits, resting on a 27 m strath surface, are constrained to be ĝ1/4 28-40 ka, suggesting an incision rate of 678-968 m Myr-1, which is a factor of 7 higher than the long-term rate of ĝ1/4 125 m Myr-1 over the past ĝ1/4 1.2 Myr. We observe fluvial deposits that were abandoned due to a bedrock meander cutoff, which partially explains our accelerated incision rate. We use a simple geometric model, informed by our field data, to demonstrate how planform river evolution may, in some circumstances, increase short-term incision rates relative to long-term averages. These short-term rates may reflect a combination of autocyclic and climatic processes, which limits their ability to resolve longer-term changes in incision rates that may be related to changes in base level or tectonics.