Feedbacks among chemical weathering, rock strength and erosion with implications for the climatic control of bedrock river incision
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Understanding the processes that erode bedrock rivers and the factors that influence erosion rates is critical to predicting the feedbacks among climate, erosion and tectonics that drive the topographic evolution of unglaciated, active orogens. However, quantitative predictions for the coupling of these feedbacks are limited because the specific mechanisms by which climate controls erosion are poorly understood. Chemical weathering, a climate-dependent process, has been suggested to play a role in the erosion of bedrock rivers, but this idea has largely lacked supporting field or laboratory data. In this dissertation I present field data collected across an orographic precipitation gradient on the Kohala Peninsula of the Big Island of Hawaiʻi. This data demonstrates that the measured rock strength in bedrock river beds is a function of both climate-dependent chemical weathering and abrasional wear. Furthermore, accounting for the effect of chemical weathering on rock erodibility improves the predictions of long-term river profile evolution across Kohala Peninsula. Additional field data was collected and compared to previous experimental data of bedrock erosion in order to explore the feedbacks among chemical weathering, fluvial abrasion and topography at the scale of bed roughness. Finally, inspired by the findings of the field data, I developed a nonlinear dynamical model that finds quantifiable, predictive relationships for changes in rock strength as a function of chemical weathering and fluvial abrasion. The findings in this dissertation demonstrate that the erodibility of bedrock rivers can be influenced by the mechanism of climate-dependent chemical weathering, and that spatial distributions of rock erodibility that develop due to the interactions of chemical weathering and fluvial abrasion can influence the morphology of bedrock rivers from the scale of bed roughness to entire stream profiles.