Sediment budget as evidence of land-use changes in mountainous areas : two stages of evolution

Two sediment budgets, for two widely different hydrological settings, have been prepared, compared and analysed to determine altered hydrological conditions resulting from land-use changes, over extended time periods. French alpine rivers are currently undergoing severe channel entrenchment. This has resulted from the accumulated effects of anthropogenic activity and climatic factors. Bridges and dikes are threatened by the entrenchment, which is the consequence of a major decline in hillslope erosion that began toward the end of the 19th century. On the other hand, in the Western Sierra Madre (northern Mexico), river channels have been rising for some decades, and at the same time widening; this is a consequence of severe hillslope erosion, which is both linear and laminar, and mainly due to over-grazing and deforestation. Both cases clearly demonstrate the impact of anthropogenic activities and, to a lesser extent, climate change, on hydrological conditions.