Comment on egusphere-2024-71

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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 retreating 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 32 m strath surface, are constrained to be ∼28-40 ka, suggesting an incision rate of 804-1151 m Myr -1 , nearly an order of magnitude higher than the long-term rate of ∼140 m Myr -1 over the past ∼1.2 Ma. 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 incision rates. These short-term rates may also reflect a combination of autocyclic and climatic processes, which limits their ability to resolve longer-term changes in incision rate that may be related to changes in base-level or tectonics. The landscape of the Colorado Plateau in the American Southwest is known for deep canyons, broad plateaus separated by steep escarpments, and mesas that protrude from otherwise low-relief surfaces. Regional denudation and the resulting landforms are closely tied to the rate of incision by large rivers, which serve as major conduits for mobilized sediment. Despite the centrality of the Colorado River and its tributaries to understanding the complex geologic, tectonic, and climatic history that shaped the 1