The Sediment Yield of High Mountain Environment Watersheds: Strong Evidence of Transport Capacity Limitation

Abstract:

Alpine landscapes are likely to be particularly sensitive to climate change, because of: (1) the vulnerability of permafrost and glacial and nival processes to changes in atmospheric temperature and precipitation; (2) their history, which may have led to high rates of sediment production, and hence high rates of sediment supply; and (3) the steep slopes that may sustain sediment transport. Assessing the effects of climate change upon these processes over the timescale of recent rapid warming (i.e. decades) is difficult because of the lack of available data. Here, we use two unique data sources to assess the relative importance of these three processes for two high mountain basins (altitude range 2’000 to 3’500 m asl): (1) histories of surface change from the late 1950s, based upon construction of digital elevation models from archival imagery; and (2) a long term record, beginning in the early 1960s, of sediment export based upon the flushing of sediment from hydropower intakes. The two basins contain glaciers, rock glaciers, permafrost and the legacy of past glacial activity. These data show that the level of geomorphic activity within both basins is orders of magnitude higher than the volume of sediment exported from the basins. Decadal scale changes in elevation and surface displacement velocities suggest an acceleration of landscape dynamics from the 1980s. Changes in atmospheric temperature remain a key control of this process, but precipitation and snow cover prove critical in controlling rock glacier behaviour and can induce acceleration of surface displacements even under cold atmospheric conditions. However, the analysis of the sediment export data show that despite generally greater within basin dynamics, the rate of export of sediment remains orders of magnitude lower than rates of sediment production. Calculations of sediment transport capacity suggest that this is because the fundamental limit upon sediment yield is the capacity of the stream to evacuate sediment that, in turn, is highly dependent upon annual winter snow cover. There is no clear trend in the latter suggesting that warming enhanced sediment production does not translate into sediment export from the basins studied.