Implications of river channel self-organization for climate and landscape evolution

Colin Phillips, Postdoctoral Scholar, St. Anthony Falls Laboratory, University of Minnesota

Evaluating the role of meteorological climate (storms) in the evolution of mountain landscapes requires understanding how coarse-grained rivers remove eroded rock and debris via water-driven sediment transport. Short hydrological records and limited observations preclude rare events making it difficult to determine the role of a changing climate on the frequency and magnitude of sediment transporting floods. To understand the effects of extreme events on river sediment transport we focus on a detailed case study of sediment motion and channel geometry in the Mameyes River located in Northeast Puerto Rico, a river with frequent flash floods and extreme discharge variability. Sediment transport and channel surveys suggest that despite large floods the river channel remains adjusted to be near the threshold for coarse sediment motion. We demonstrate that the results from the Mameyes River are general by analyzing channel geometry and stream-flow records from 186 coarse-grained rivers across the United States. We find that channels adjust their geometry such that floods slightly exceed the stress required to transport bed sediment – regardless of widely-varying climatic, tectonic, and lithologic controls. Remarkably, the distribution of fluid stresses associated with floods is consistent, indicating that self-organization of near-critical channels filters the climate signal evident in discharge. This effect blunts the impact of extreme rainfall events on landscape evolution. Coupling these findings with recent experimental results suggests a simplistic treatment of climatic forcing in long-term mountain landscape evolution models.