GLACIAL AND FLUVIAL VALLEY EXCAVATION IN BASINS OF THE NORTH FORK BOISE RIVER

The incision of valleys by glaciers and rivers is a key process in the evolution of mountain drainage basins. The degree to which glacial erosion serves to enhance or reduce relief has been the subject of considerable recent research; with some studies finding evidence for enhancement of relief, while others have found evidence for reduction in relief. Preliminary observations suggest there are differences in the amount of cumulative valley excavation by glacial and fluvial processes within subbasins of the North Fork Boise River in central Idaho. Field observations and DEM analysis suggest that subbasins historically dominated by glacial processes are confined to the easternmost portion of the basin, while subbasins in the western portion have been dominated by fluvial processes. Visual inspection of fluvial and glacial basins on topographic maps reveals that while the familiar narrow-fluvial valley and wide-glacial valley comparison can be made on several scales, relative relief comparisons are more difficult to generalize. There are both high and low-relief valleys in both fluvial and glacial basins. Thus, it is difficult to make comparisons between excavation volumes and scale of relief production by fluvial and glacial processes without quantitative analysis. By using cross-sectional areas derived from DEM's in both fluvial and glacial valleys we aim to evaluate potential differences in the volume of material excavated by glacial and fluvial processes. In particular we aim to determine whether alpine glaciers are more effective agents of erosion than rivers in drainage basins where both process occurred simultaneously, albeit in adjacent locations.