ASSESSING THE RELATIVE EFFICIENCY OF FLUVIAL AND GLACIAL EROSION: EXAMPLES FROM THE SIERRA NEVADA, CALIFORNIA, AND SANGRE DE CRISTO RANGE, COLORADO

Glacial erosion forms a key component in the denudation of mountain belts, especially when the role of climate change is questioned. We present clear evidence that temperate valley glaciers have been more effective agents of erosion than rivers above the mean Quaternary equilibrium line altitude (ELA). As suggested by Brozovic et al. (1997), glaciers seem to limit regional elevations in these field areas. Our analysis is based on direct comparison of topography between basins that have experienced different degrees of glacial modification. We calibrate a fluvial erosion model by using slope-area analysis in the nonglaciated basins to extract mean estimates of channel steepness and concavity. As a quantitative estimate of predicted modern topography in the circumstance that glaciation had not occurred we use the mean steepness and concavity estimates to simulate the topography of the glaciated basins (1-D longitudinal profiles and 2-D basin simulations [GOLEM: Tucker & Slingerland, 1994, 1997]). This allows direct comparison between predicted fluvial topography and the modern (glaciated) topography via a digital elevation model. This comparison indicates that glaciers have effectively "planed off" topography above the mean Quaternary ELA, carving flat-floored cirques and troughs and apparently lowering peaks and ridges by a similar amount (only minor changes in net relief). Below the ELA, glaciers have widened valley floors but have had little impact on the longitudinal profile. Our 1-D simulations have also led to the development of a simple method for estimating glacial (horizontal) headwall erosion rates. Our results from Sierra Nevada have been promising, and consistent with independent field evidence, while in the Sangre de Cristos we have enjoyed some success but also clarified some of the limitations of this approach. Our observation of glacial headwall erosion suggests the possibility of drainage reorganisation by glaciers. With a quantitative understanding of the changes in erosivity of the fluvial system during the Quaternary, coupled with a detailed knowledge of baselevel and tectonic histories, our approach could be extended to estimate glacial downcutting rates.