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Variation in bedrock erodibility along a river profile gives rise to differences in vertical incision rate and influences sediment characteristics such as clast lithology, coarse sediment generation rate, and grain size. In rivers whose beds are eroded, in part, through sediment abrasion, these streamwise sediment dynamics are part of a crucial feedback that sets the dominant fluvial erosion process and determines whether a river exhibits transport-limited or detachment-limited behavior. The role that sediment plays in setting the shape of a river profile is of particular interest in the case of a river’s transient response to external forcing. Here we present a model that explores river profile evolution in a setting with streamwise bedrock variability. Our model combines theory for five interrelated processes: bedload sediment transport in equilibrium gravel-bed channels, channel width adjustment to flow and sediment characteristics, abrasion of bedrock by mobile sediment, plucking of bedrock, and progressive loss of gravel-sized sediment due to grain abrasion. We envision a generic “range-foreland” system that consists of erosion-resistant, crystalline rocks in the upstream reaches, juxtaposed with softer, more erodible rocks downstream. In this setting, coarse sediment generation is confined to the upstream part of the fluvial system. As the sediment is transported downstream, it creates an alluvial blanket across the soft, fine-grained unit. Bedrock erosion is modulated by the thickness of the alluvial layer. We use the model to explore the range of transient forms that can occur in such a setting in response to changes in tectonic or climatic regime. We pay special attention to the conditions under which the upstream gravel source either increases the downstream fluvial gradient (by partially shielding the underlying material from incision) or decreases the gradient (by providing tools that amplify the efficiency of abrasion). We also examine the conditions under which erosion is concentrated at the downstream-most reaches of the river profile, versus at the lithologic boundary. While our work takes its motivation from the Southern Rocky Mountains and High Plains of North America, the model is applicable generally to settings in which a bedrock-incising river traverses multiple lithologies. This work aims to improve our interpretations of the history of river profiles in lithologically heterogeneous environments and inform our understanding of landscape evolution in these settings.

vanessa.gabel@colorado.edu

Geolology Graduate Student, ���Ҵ�ý�ƽ������