Regional assessment of the multi-decadal changes in braided riverscapes following large floods (Example of 12 reaches in South East of France)

摘要

The district of the France Rhône basin is characterised by severalbraided reaches, preserved from the widespread disappearing occurred in the20th century. Even if the overall pattern is evolving through a generalriver narrowing, some reaches have shown to be still active and havewidened. The preliminary results suggest that differences in width patterncould be related to several factors, such as high magnitude and lowfrequency floods, and geographical position in the catchment whichinfluences bedload delivery conditions and vegetation recruitment related toclimate. From an initial set of 53 braided reaches, we selected 12 sites,distributed into four main hydro-geographical regions. Reaches were selectedto be representative of the overall study area. We analysed the braidingwidth pattern and the vegetation pattern dynamic among five observationperiods dating from the 1950s to the 2000s. We hypothesised that acomparative analysis of a detailed temporal trajectory (i.e. five dates) ofa set of rivers within several hydro-geographical contexts would allow us tobetter distinguish the relative role of floods (in terms of magnitude andduration) and other controlling factors acting at the regional scale. Weshowed that active channel width is controlled mainly by flood andsecondarily by bedload availability whereas island pattern is in large partindependent of flood series characters. Moreover a clear regionaldifferentiation, constant over time, in terms of riverscape response isobserved, mainly opposing south-western and south-eastern reaches. Thisopposition depends on several concurring factors, i.e. the flood characters,the river activity, the human influence and the climate. Finally, thesefindings allowed us to highlight those sectors in which the braided patterncould disappear, and those sectors in which the braided pattern is stillactive, because critical processes responsible of channel dynamic are stillpresent.