Climatic, tectonic and lithologic controls on the size distribution of sediments supplied to channels: implications for transient evolution of bedrock river profiles

Recent theoretical, laboratory and field investigations strongly suggest that the bedload grain size distribution is a first-order control on river channel slope, bedrock incision rate, and the landscape response time to changes in tectonic and climatic forcing. As the size distribution of sediment supplied from local hillslopes and tributaries can dominate the bedload grain size distribution in a river, the processes generating hillslope sediments may exert a fundamental control on channel dynamics. Although few data are available to constrain estimates of the sediment distribution supplied to channels, we propose several broad hypotheses to guide exploratory modeling: Coarse size distributions will be favored by stronger rocks, colder and dryer climates, and by more rapid rates of rock uplift and landscape denudation; finer size distributions are favored by the opposite conditions. Here we combine models for bedrock incision by saltating bedload and the topographic dependence of precipitation and temperature with heuristic relations for the grain size distribution supplied by hillslopes to channels, to explore the sensitivity of river longitudinal profiles to variations in climate, rock strength and rates of relative baselevel lowering. We use two metrics to represent sediment size distributions, the fraction of total supply greater than a threshold size (2mm) and the median size of this coarse, bedload-size fraction. Preliminary model predictions suggest the potential for both positive and negative feedbacks in transient profile evolution. For example, accelerated rock uplift creates elevated topography favorable to cooler conditions and more efficient mechanical weathering and coarse sediment production. Increased supply of coarse sediment causes local channel steepening and greater overall profile relief, reinforcing the elevation-driven cooling effect. Conversely, enhanced orographic precipitation favors more efficient chemical weathering and fine sediment production. However, wetter conditions, combined with more rapid relative base level lowering, may also lead to increased landsliding and enhanced supply of bedrock-derived rock fragments directly to channels. Many intriguing questions emerge from this work, including the response time of hillslopes to changes in weathering regime, the relative importance of soil production and transport versus landsliding in delivering coarse sediment to the channel, and climatically- sensitive biological influences on rates and styles of coarse sediment production and weathering on hillslopes.