Meteorological and land use controls on past and present hydro-geomorphic processes in the pre-alpine environment : an integrated lake-catchment study at the Petit Lac d'Annecy, France

Abstract

A wide range of environmental records is integrated in order to reconstruct the mechanisms of flooding and sediment transport within the 170 km(2) Petit Lac catchment, Annecy, France, over time scales of 10(-1) to 10(2) years. These records include sequential lake sediment trap samples and cores, floodplain stratigraphies, dated landform assemblages, hydro-meteorological records, and documented histories of river channel and land-use change. Mineral magnetic measurements are used as the basis for classifying catchment sediment sources and tracing sediment movements through time. Records of magnetic susceptibility for monthly sediment trap samples (1998-99) track seasonal discharge, peaking in winter and spring. Magnetic records in lake sediment cores are compared against and tuned to precipitation records to provide dated proxy records for past discharge spanning sub-annual to decadal time scales back to 1826. Calculated sediment accumulation rates in lake sediment cores are used as proxies for time-averaged catchment sediment load. Analysis of the results reveals that climate and land-use controls on the hydrological and sediment system are complex and vary according to the time scale of observation. In general, cycles of agricultural expansion and deforestation appear to have been the major cause of shifts in the sediment system through the late Holocene. Deforestation in the 18th century may have caused a number of high-magnitude flood and erosion events. As the time scale of observation becomes shorter, changes in climate and hydro-meteorological conditions become progressively more important. Since the mid-19th century, smoothed records of discharge roughly follow annual precipitation; this is in contrast to sediment load, which follows the trend of declining land-use pressures. Episodic erosion events during this recent period seem to be linked to geomorphic evidence for slope instability in the montane and sub-alpine zones, triggered by intense summer rainfall. At the annual scale, changes in seasonal rainfall become paramount in determining sediment movement to downstream locations. The study demonstrates that the connections between forcings and responses span a four-dimensional array of temporal and spatial scales, with strong evidence for dominantly nonlinear forcing-response mechanisms. Copyright (C) 2003 John Wiley Sons, Ltd.