Contemporary changes in sediment yield in an alpine mountain basin due to afforestation (the upper Drôme in France)

Abstract

The Baurières plain within the upper Drôme River basin was used to reconstruct recent changes in sediment supply in relation to changes in land use within an alpine catchment. A considerable body of archival information is available. Furthermore, the plain acts as a natural sediment trap and the reach–basin interaction has not been disrupted by human activity. Based on archival data, channel geometry measurements, dendrogeomorphological and radionuclide analysis (Cs 137 and unsupported Pb 210), the trends in channel change and sediment supply over the past two centuries are assessed and their causes are interpreted. Dendrogeomorphology and radionuclide profiles show that the floodplain is characterised by a decrease in sedimentation rate in the 1960s. The ex-Pb 210 profiles also suggest a spatial modification of the relative contribution in sediment supply of the catchment. Bedload yields are estimated to be at least 26 m 3 km −2 year −1 between 1928 and 1996 based on an estimate of storage over a channel length of 11.5 km. Archival data concerning bedload removal from traps yields an estimate of 25.6 m 3 km −2 year −1 between 1993 and 2001. These two values are very comparable suggesting no major modification in bedload transport within the reach during the 20th century. If the bedload supply has not strongly decreased in the studied reach during this period, the bedload sources have changed. The volume of sediment stored in the Beaurières area between 1928 and 1996 corresponds to at least 40% of the sediment delivered by channel degradation from an upstream alluvial reach. Both changes in floodplain sedimentation as well as changes in bedload and suspended contributions from catchment sources are interpreted as responses to land use but also flow regime. Hillslope sediment production strongly decreased due to planned hillslope afforestation and torrent regulation at the end of the 19th century, and spontaneous hillslope afforestation resulting from grazing decline, mainly in the two decades following World War II. The observed change in suspended sediment supply which occurred around 1960–1965 has a clear synchronicity with spontaneous catchment afforestation following World War II. After this period, a decrease in sediment supply, a change in source, but also a decrease in peak flow, were observed. Changes in run-off are complex and cannot be caused with only land use change. Flood hydrographs underwent peak decreases and duration increases through the 20th century because of increase in water retention capacity of the forested catchment. In addition flood seasonality has changed, with September and October flood events being much less frequent in the last part of the 20th century.