Effect of short-term, climate-driven sediment deposition on tectonically controlled alluvial channel incision

Debate about relations between rates of fluvial incision and time (the “Sadler effect”) continues, impeding the use of incision rates to infer tectonic and climatic processes. There is a dearth of detailed field evidence that can be used to explore the coupling between tectonics and climate in controlling alluvial channel geometry and incision rates over time scales of 102–105 yr. We present field data from the Rumei watershed of southeast Tibet, which we obtained by mapping and dating late Pleistocene (ca. 135 ka) fluvial terraces and related channels, measuring channel hydraulic geometry, and calculating channel steepness indexes and incision rates. The evidence indicates that climate forcing is the main driver of sediment production and delivery to streams in the watershed. New aggradation events altered alluvial valley and channel geometry and, coupled with tectonic uplift, affected the rate of channel incision in the catchment. We propose a conceptual model (Formula presented.) that links uplift-driven incision (II) to channel aggradation (Formula presented.)induced by climate change, which is valid in catchments and other areas. We conclude that the reduction in incision depth caused by climate-driven channel aggradation is significant on short time scales (102–105 yr), and its cumulative effect contributes to the “Sadler effect” on long time scales (>106 yr).