COARSE SEDIMENT SETS THE SLOPE OF BEDROCK CHANNELS IN A FAST UPLIFTING TERRAIN, COASTAL RANGE OF EASTERN TAIWAN

It has been shown that incision rate and steepness of steady-state bedrock channels are controlled by uplift rate, substrate properties, coarse-sediment flux, and bedload grain-size. However, the relative role of these factors, and whether there is a threshold of channel steepness at fast uplift rates (>1 mm/yr), remain unclear. The Coastal Range in eastern Taiwan is one of the fastest uplifting terrains in the world where steep river catchments developed on both resistant volcanic basement (mean uniaxial compressive strength (UCS) = ~39.3 MPa) and weak sedimentary rocks (mean UCS = ~13.9 MPa) have been rapidly raised above sea level in the past ca. 1 Ma. We conducted quantitative topographic analysis of carefully selected reaches with varying substrate lithology and sediment source rock along the east coast of the Coastal Range, spanning a wide range of uplift rates (1.11 to 13.5 mm/yr) and evenly distributed precipitation (1.8 to 2.5 m/yr), to evaluate the primary controls on channel steepness. We also adapted the saltation-abrasion model (Sklar and Dietrich, 2004; 2006), the only mechanistic model for bedrock incision, with field-constrained parameters to independently assess the proportion of slope that may be limited by coarse sediment effects. Our results show that channel steepness is independent of uplift rate but systematically varies with bedload grain-size and bedrock lithology. Slope component analysis shows that the threshold of sediment motion and flux of coarse sediment appear to exert the dominant control on setting the slope of bedrock channels in areas undergoing such high rates of uplift (> 1 mm/yr). These findings imply that high flux of coarse sediment associated with an increase in mass-wasting products from unstable hillslopes, where the gradient has reached the angle of repose, may govern the slope of bedrock rivers by a combination of enhanced bedrock wear (tool effect) during floods as well as inhibited incision (cover effect) during standard days. For a given bedrock strength, the value of channel steepness can become insensitive to uplift rate and reach “saturation” in fast-uplifting terrains due to the dominant effects of coarse sediment size and flux.