Climatic variation and runoff in mountain basins with differing proportions of glacier cover*

Records of discharge from partially-glacierised basins in the upper Rhône catchment, Switzerland, were examined together with air temperature and precipitation data in order to assess impacts of climatic fluctuation and percentage glacierisation of basin on runoff, as glaciers declined from dimensions attained during the Little Ice Age. Above 60% glacierisation, year-to-year variations in runoff mimicked mean May–September air temperature, rising in the warm 1940s, declining in the cool 1970s, before increasing (by 50%) into the warm dry 1990s/2000s but not reaching 1940s maxima. In basins with between 35–60% glacierisation, flow also increased into the 1980s but waned through the 1990s. With less than 2% glacierisation, the pattern of runoff was broadly the inverse of that of temperature and followed precipitation, dipping in the 1940s, rising in the cool wet late 1960s, and declining into the 1990s/2000s, with glacier melt in warm years being insufficient to offset lack of precipitation. On mid-sized glaciers at relatively low elevations and with limited vertical extent, in warmer years, the transient snow line was above the highest point of the glacier. Only on large glaciers descending from high elevations can rising transient snowlines continue to expose more ice to melt. Runoff from such large glaciers was enhanced in warm summers but reduction of overall ice area through glacier recession led to runoff in the warmest summer (2003) being lower than the previous peak discharge recorded in the second warmest year (1947).