POST-GLACIAL FLUVIAL INEFFICIENCY

This study examines the effect of topography to limit the efficiency of fluvial erosion in a post-glacial landscape by quantifying the post-glacial sediment budget from source to sink in Moran and Snowshoe Canyons in the Teton Range (Wyoming, western USA). The volume of sediment in Moran and Snowshoe Canyons was estimated using a combination of 1) differencing a modeled bedrock digital elevation model from the surface topography for a portion of Moran Canyon, 2) calculating the colluvial sediment production rate and applying this to the rest of the catchment and 3) assigning thickness based on landform observations. Sediment from the canyons is preserved in Moran Bay which is isolated from the rest of Jackson Lake by a submerged Pinedale age (14.4 ka) moraine. Four CHIRP seismic profiles were acquired, and an isopach map and sediment volume estimate were produced using Seisware and Trinity T3 software. We find that only about 2% of the total sediment volume has been transported into the bay. Although both fluvial and glacial processes can potentially keep pace with rapid tectonic uplift, an efficient erosional system is required, and the topography determines the erosion rate for a given process. During interglacial periods the over-steepened hillslopes produce abundant colluvial sediment, but the rivers in these glacially flattened valleys are incapable of transporting it. Glacial erosion rates are high while adjusting from fluvial to glacial conditions, but once valleys are equilibrated for glacial erosion, subsequent glaciers may not need to erode much bedrock but mostly transport the accumulated hillslope sediment to maintain the valley shape and equilibrium between denudation and uplift.