Annualmeeting:2017 CSDMS meeting-044
Understanding River Terrace Formation and Destruction, Channel Lateral Mobility, and River Valley Widening from Base Level Fall Experiments
[[Image:|300px|right|link=File:]]Fluvial terraces are commonly interpreted as recorders of past environmental (e.g. tectonic or climatic) conditions. However, controls on terrace formation through river incision, and on the destruction of terraces through lateral erosion are poorly understood. Here, we present results from a physical experiment performed at the St. Anthony Falls Laboratory that provide insights into the formation and preservation potential of alluvial terraces, into dynamics of alluvial valley width, and the dependence of these parameters on external forcings: primarily on river response to base level fall. The model was performed in a wooden box with dimensions of ~4 meters by ~2.5 meters by ~0.5 meters, which was filled with silica sand with a unimodal grain size distribution (D50= 0.14 mm). Sediment and water were mixed and fed into the box via a gravel diffuser to inhibit scour. A single channel incised down to the base level, which was steadily lowered by a weir. Six experiments were performed, each with a constant water discharge of 0.1 L/s and a sediment flux of 0.022 L/s, and with a base-level fall rate of 0mm/hr, 25mm/hr, 50mm/hr, 200mm/hr, 300mm/hr, and 400mm/hr. We collected aerial photographs every 20 seconds and digital elevation models (DEMs) every 15 minutes throughout each experiment. Terraces formed in the experiments with base level fall due to incision and headwards knickpoint retreat. Major sidewall collapses and progressive valley widening were observed and controlled by the lateral migration of the channel.