INSIGHTS INTO THE DEVELOPMENT OF GLACIAL LANDSCAPES FROM DIGITAL ELEVATION MODEL ANALYSIS AND LANDSCAPE EVOLUTION MODELS

Considerable insight into the development of glacial landscapes has been gained through a detailed, quantitative comparison of fluvial and glacial landscapes in the western US and New Zealand. The eastern Sierra Nevada, California, the western Sangre de Cristo Range, Colorado, and the western Southern Alps, New Zealand, exhibit drainage basins spanning a full range of glacial modification, from those with essentially no modification by glaciers, to those where glaciers have carved U-shaped cross-sections and deposited moraines to the range front. This allows both direct comparisons amongst these basins, and local calibration of a fluvial landscape evolution model to suggest how the landscape might look now had glaciers not developed. Digital elevation models (DEMs) have been analyzed by extracting longitudinal profiles, determining hypsometry, and calculating relief distributions. The fluvial erosion model was calibrated using slope-area analysis in the nonglaciated basins to extract mean estimates of channel steepness and concavity. The mean steepness and concavity estimates were used to simulate a quantitative estimate of modern fluvial topography in the glaciated basins. This indicates how these basins might look under modern climatic conditions had the glaciers never developed, and thus indicates how the glaciers have modified the landscape. The next step would be to determine how the landscape might have looked prior to the onset of glaciation. However, this requires improved understanding of how channel steepness and concavity respond to climate change, and how base-level has changed through time in these ranges. Principal conclusions include: (i) fluvial and glacial landscapes are readily distinguished using DEM analysis. (ii) glacial incision of the valley floor tends to be focused at or above the long-term equilibrium line altitude in smaller drainage basins (<12 km long), although glaciers extending to the range-front have carved out U-shaped cross-sections at much lower elevations. (iii) in larger drainage basins (~30km long) glacial incision extends to much lower elevations. (iv) headwall erosion by glaciers leads to drainage capture and relief inversion. (v) glaciers have redistributed relief, but other than basin enlargement by headwall erosion, net relief production has been modest.