Glacial erosion and relief production in the Eastern Sierra Nevada,California

The proposal that climate change can drive the uplift of mountain summits hinges on the requirement that glacial erosion significantly enhances the relief of a previously fluvially sculpted mountain range. We have tested this hypothesis through a systematic investigation of neighbouring glaciated and nonglaciated drainage basins on the eastern side of the Sierra Nevada, CA. We present a simple, objective method for investigating the relief structure of a drainage basin, which shows noticeable differences in the spatial distribution of relief between nonglaciated and glaciated basins. Glaciated basins on the eastern side of the Sierra Nevada have only ∼80 m greater mean geophysical relief than nonglaciated basins. This “extra” relief, though, is attributable principally to the larger size of the glaciated basins, as geophysical relief generally increases with basin size. The glaciers on this side of the range were only responsible for relief production if they substantially increased headward erosion rates into low relief topography, such as an elevated plateau, and thus enlarged previously fluvial basins. We carried out a preliminary morphometric analysis to elucidate the importance of this effect and found that the glaciers of the eastern Sierra Nevada may have eroded headward at considerably faster rates than rivers, but only when they were not obstructed from doing so by either competing larger glaciers in adjacent valleys or transfluent ice at the head of the basin. Our results also suggest that, in temperate regions, alpine glaciers are capable of eroding downward at faster rates than rivers above the equilibrium line altitude (ELA). Although we can rule out significant peak uplift in response to local relief production, in the special case of the Sierra Nevada the concentration of mass removal above the ELA could have contributed to flexural uplift at the edge of a tilting block.