L'incision des rivières dans les Alpes du nord : état de la question / River incision in the Northern French Alps

Abstract

This study analyses recent river-bed incision in the northern Alps (France). This incision began after the second world war and, initially, affected mainly the river channels of large alpine valleys ; incision exists also on many mountain rivers but is less extensive because of the bed-rock outcrop. The river-bed incision is very severe : at present, many rivers are incised to 3-4 metres and the long profile lowering of several rivers exceeds locally 8 to 14 metres ; rates of incision exceeding 50 cm/year were frequent during the period 1950- 1970. The channel incision results from the interaction between : a) a natural decrease of the bed- load supply after the Little-Ice- Age ; b) a reduction of bank erosion due to the longitudinal embankment of fluvial channels ; c) a control of torrent dynamics by reforestation and contruction of weirs between 1870 and 1920 ; d) a drastic change of hydrology and sediment transport after dam construction ; e) major instability of river beds due to excessive gravel extraction ; f) a trapping of the bed-load by weirs built to prevent the lowering of long profiles.The future of rivers flowing in the northern Alps relates not only to river-bed stabilization by civil engineering works but also to the systemic management of water discharge and sediment supply to restore the longitudinal connection between the different reaches of rivers.