BEDROCK RIVER MORPHOLOGY REFLECTS UPSTREAM SEDIMENT SUPPLY IN RAPIDLY UPLIFTING SOUTHERN TAIWAN (Invited Presentation)

In tectonically active landscapes, rivers generate relief by exporting sediment released by hillslope processes and eroding underlying bedrock. The fluvial transport capacity and incision potential must achieve both or end up filled with hillslope debris or perched high above baselevel. How river morphology evolves to accommodate these drivers is a topic of open research, yet tectonic geomorphologists often assume a detachment-limited framework in which channel morphology reflects local rock-uplift while upstream sediment supply has little to no impact on morphology. Because tectonic uplift and river incision ultimately drives the supply of sediment from hillslopes, it remains a difficult yet important challenge in tectonic geomorphology to disentangle the roles of upstream sediment supply and local tectonic uplift in controlling bedrock channel geomorphology. The landscape response to the tectonics of southern Taiwan offers an opportunity to quantitatively tease apart these controls. A southward propagating orogen sets up a scenario in which river incision outpaces hillslope erosion and sediment supply as valley relief grows with little change in lithology and climate along a southward transect. To capitalize on this natural experiment, we measure rates of river incision in 14 basins using ages of strath terraces determined by quartz OSL and feldspar IRSL geochronology. We combine these estimates with previously measured hillslope erosion rates to compare with local channel morphological signatures. Rates of river incision vary along strike by ~3 fold (~1 to 3 mm/yr), whereas hillslope erosion rates vary by over an order of magnitude (~0.1 to 4 mm/yr). Regression analysis of log-transformed data suggests local channel steepness more closely tracks upstream sediment supply (r²>0.7) than local incision rate (r²<0.3). The results contest the assumption that bedrock channel behavior most strongly reflects local rock-uplift rate rather than upstream sediment supply, an assumption that pervades the bulk of tectonic geomorphology literature. These findings have implications for understanding tectonic-climate interactions, the topographic signature of tectonic hazards, and how we model landscapes in tectonically active regions.