Controls on sediment evacuation from glacially modified and unmodified catchments in the eastern Sierra Nevada, California

Abstract

The degree of glacial modification in small catchments along the eastern Sierra Nevada, California, controls the timing and pattern of sediment flux to the adjacent fans. There is a close relationship between the depth of fan-head incision and the pattern and degree of Late Pleistocene catchment erosion by valley glaciers; catchments with significant glacial activity are associated with deeply incised fan heads, whereas fans emerging from glacially unmodified catchments are unincised. We suggest that the depth of fan-head incision is controlled by the potential for sediment storage during relatively dry ice-free periods, which in turn is related to the downstream length of the glacially modified valley and creation of accommodation through valley floor slope lowering and glacial valley overdeepening and widening. Significant storage in glacially modified basins during ice-free periods leads to sediment supply-limited conditions at the fan head and causes deep incision. In contrast, a lack of sediment trapping allows quasi-continuous sediment supply to the fan and prevents incision of the fan head. Sediment evacuation rates should thus show large variations in glacially modified basins, with major peaks during glacial and lows during interglacial or ice-free periods, respectively. In contrast, sediment removal from glacially unmodified catchments in this type of setting should be free of this effect, and will be dominated instead by short-term variations, modulated for example by changes in vegetation cover or storm frequency. This distinction may help improve our understanding of long-term sediment yields as a measure of erosional efficiency.