The Threshold of Motion Filters Extreme Climatic Fluctuations in Gravel Bedded Alluvial and Bedrock Rivers Resulting in Near-Threshold and Transported-Limited Systems

Abstract:

Climate is a primary driver of mountain landscape evolution, and recent research indicates that hydrologic variability – characterized by the distribution of flood magnitudes – may control mean mountainous landscape erosion rates. The connection between precipitation and river incision is regulated, however, by coarse-grained bed load transport and dynamic channel adjustment. The nature of these connections has been explored with numerical models that arrive at sometimes conflicting conclusions. Here we show that the critical stress to initiate bed-load transport is a filter for extreme flood events, acting to decouple the distribution of fluid stress in a river from its regional climate and discharge. Results are drawn from a detailed study of a tropical-montane bedrock river with extreme discharge variability, and from a compilation of stream-flow data from gravel-bedded bedrock and alluvial rivers across the United States. Discharge distributions vary widely in their form depending on geographic context, we find that the distributions of fluid stress above critical have a narrow range and all follow a universal exponential decay with a mean value consistent with analytical predictions. We demonstrate that channel geometry in each river is adjusted to a characteristic flood that coincides with the bankfull depth. Two important conclusions are: (1) bedrock channels are near-threshold transport-limited systems, similar to alluvial gravel rivers, providing a useful constraint for models; and (2) the most important control of climate is the frequency of flows above critical. Results call into question the significance of extreme events in determining longterm landscape evolution, and suggest the importance of grain size as a controlling parameter on channel geometry and river incision rates.