THE EFFECTS OF INCREASED FLOW RATE ON EROSIONAL PROCESSES IN A YOUNG, EVOLVING WATERSHED

Throughout the world, land use changes are having a dramatic impact on how water enters river systems, altering flow regimes. We are using a small experimental basin to explore how increases in flow rate impact erosional rates and processes in a young, evolving landscape. We report here on a series of experiments that use sheet flow across an erodible bed in a 0.5 m by 0.5 m basin to form a river network following a single base level drop. As water flows over the surface toward the outlet a knickpoint forms, and the watershed evolves as this knickpoint migrates upstream. This mirrors landscape evolution processes in a tectonically inactive region in central Minnesota, where post-glacial incision on the Minnesota River triggered knickpoints now migrating upstream on each of the Minnesota River’s tributaries. In addition to the natural landscape adjustment, urban and agricultural drainage coupled with on-going climate change have caused measurable widening to occur in these major tributaries over the last 70 years. This widening results in significant sediment contributions to the turbidity-impaired river and has taken place in spite of conservation practices meant to reduce erosion in agricultural fields. The experiments we are reporting on quantify how sensitive an evolving landscape is to changes in flow regime at the watershed scale, and how changes in flow delivery affect erosional rates, processes, and spatial distribution.