RELATIVE EFFICACY OF GLACIAL VERSUS FLUVIAL EROSION IN THE UINTA MOUNTAINS, NORTHEASTERN UTAH

Late Pleistocene alpine glaciers in the east-west trending Uinta Mountains were most extensive at the western end of the range; the maximum size of glaciers decreased in each valley eastward across the range. Carter Creek on the north flank and Ashley Creek on the south flank are the easternmost glacially eroded valleys. These valleys and those to the west were carved by a combination of fluvial processes and major alpine glaciations; valleys to the east were carved exclusively by fluvial processes. The physical characteristics—drainage basin sizes, elevations, valley orientation, bedrock lithologies, and hydrometeorology—of valleys in the Uintas are very similar to adjacent valley on the same flank, except for the late Pleistocene history of valleys to the west having been glaciated and the valleys to the east remaining unglaciated.

To evaluate the efficiency of glacial versus fluvial processes in excavating material, over 100 cross-valley profiles from both glaciated and non-glaciated drainages were constructed orthogonally to valley orientation in ArcView GIS from a digital elevation model of the Uinta Mountains. Valley width, ridge-crest-to-valley-bottom relief, and cross-sectional area were used to evaluate differences in mass of rock and sediment excavated below ridgelines. For comparison of net erosion from glacial versus fluvial processes, profiles were normalized using up-valley drainage area.

For the smaller drainages (< ~10 km²) that were measured on the north flank of the range, glaciated drainages exhibit significant differences compared to the non-glaciated drainages. The normalized relief of glaciated valleys becomes as much as 50 % greater than non-glaciated valleys and normalized cross-sectional area of glaciated valleys becomes three to four times larger than adjacent non-glaciated valleys. For the larger drainages (> ~25 km²) that were measured on the south flank of the range, these same relations are apparent. The larger valley cross sections and greater ridge-crest-to-valley-bottom relief found in the glaciated versus non-glaciated drainages indicate that in this sub-alpine setting glaciers are far more effective erosive agents than rivers.