LITHOLOGY ERODIBILITY AND CHANNEL CROSS-SECTIONAL GEOMETRY CONTROL THE EVOLUTION OF MEANDERING BEDROCK RIVERS IN UPLIFTED OROGENS

Meandering bedrock rivers occur all over the planet. Due to lack of a thick sediment cover, meandering bedrock river channels are sensitive to climatic and tectonic changes that are directly interacting with the channel either by inducing flooding, altering sediment supply, and/or uplifting the channel. This unique characteristic makes bedrock meandering rivers actively incise and reshape the Earth’s landscape. The rate and style at which bedrock river channels migrate or erode are strongly dependent on the substrate of the river; therefore, lithology is hypothesized to control the landscape evolution of bedrock meandering rivers. The goal of this study is to utilize a newly developed 1-D numerical model to examine the lithologic control on bedrock meandering rivers. Our model tracks channel cross-sectional shape as a control on water depth and channel width. Lithologic controls on the stability of meandering bedrock rivers – lateral erodibility, vertical erodibility, cross-sectional shape, and initial slope – are varied in the model simulations, which run for 100,000 model years to simulate distinct landscape evolution patterns. The patterns are then compared with real-world river channels, namely the Smith River, Oregon, USA. We find that a single-thread meandering bedrock river can evolve into a straight channel, meandering channel, or braided channel depending on the balance between lateral and vertical erosion rates and channel geometry. While the ratio of lateral to vertical erosion rates influences whether the simulated channel is single or multi-thread, the formulation for channel geometry is a first-order control, thus highlighting the need to study the evolution of channel cross-sections in bedrock rivers. The behavior of the model is supported by direct field evidence from the Smith River.