Modeling the influence of rainfall gradients on discharge, bedrock erodibility, and river profile evolution, with application to the Big Island, Hawai'i

Motivated by convex-concave bedrock river profiles developed across a climate gradient on the wetside of the Kohala Peninsula of the Big Island of Hawai'i, we numerically model how rainfall gradients may influence longitudinal fluvial incision patterns. First, we model transient profile adjustment with two tectonic boundary conditions: subsidence and uplift. In this generalized analysis, we assume that rainfall gradients only influence incision by modifying the relation between upstream drainage area and local discharge. Using a detachment-limited model, downstream increases in rainfall lead to profile convexities during transient adjustment in both tectonic settings, and this is the opposite of the predicted increase in profile concavity that would develop in a steady state uplifting profile. A transport-limited erosion model develops only concave channel profiles without clear signatures of the rainfall pattern. Second, we model the development of convex-concave transient profiles and incision patterns on Kohala using a detachment-limited model. If rainfall gradients only influence incision through the local discharge, reasonable rainfall gradients can only develop channel convexities that are much smaller than those observed. Instead, we hypothesize that local bedrock erodibility increases with the degree of rainfall-dependent chemical weathering. When local erodibility is assumed to scale with local rainfall rate, the model can produce convex-concave profiles similar to those observed in Kohala. Our results suggest that changes in local bedrock erodibility due to local climate-dependent weathering may be an important mechanism by which climate influences landscape form and rates of evolution. This hypothesis requires further testing in this study area and beyond.

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