LONG PROFILE RESPONSES OF ALPINE BRAIDED RIVERS IN SE FRANCE

Channel elevation change since the early 20th century was characterized for 31 alpine braided river reaches (total length of 129 km), representing about 20% of the braided river network length in SE France. Present-day long profiles were surveyed in the field and compared with historical profiles measured between 1894 and 1930. Forty-four cross sections of active channels including terrace boundaries were also surveyed to study morphological signatures of braided channels characterized by different channel responses (aggradation or degradation). Results show that 56% of the sampling river length is characterized by degradation, 24% by stability and 20% by aggradation. Gravel mining is recognized as the main factor explaining degradation of braided channels. It is also demonstrated that aggrading braided channels are those where sediment supply from active torrents and bank erosion have been best preserved. The comparison of morphological signatures associated with aggrading and degrading channels reveals significant differences in cross-section morphometry. A normalized bed relief index is proposed as a reliable indicator of the channel response. Field observations show that long-term aggraded braided channels have a lower bed relief index than degraded channels, as already established by some laboratory experiments. This index can be used to determine the position of the braided channel in the cycle of degradation-recovery following the cessation of gravel mining. It is also established that aggraded channels develops wider active channels than degraded channels for a same drainage area. This study provides new insights for the assessment of spontaneous recovery potential of braided channels impacted by gravel mining. Copyright © 2012 John Wiley & Sons, Ltd.