Regional variability of aquatic pattern in braided reaches (example of the French Rhone basin)

We analysed, at a regional scale, the braiding pattern and the structure of the water channel habitats of 53 braided reaches located in the Rhne hydrographical district (France), in relation to three control factors: (i) spatial location along the network (i.e. slope, altitude), (ii) hydrological conditions and (iii) morphological conditions. This research is based on aerial orthophotos belonging to the French National Geographic Institute (IGN) taken between 2002 and 2006. We defined a regional typology of braided reaches based on the distribution of the variables listed above, and identified five hydro-geographical types of braided reaches. Following this regional classification, we compared the pattern of braided reaches by testing several parameters, which describe the physical characters of water channel habitats (such as braided index, channel sinuosity, or aquatic habitat diversity). We found that discharge appears to be less relevant than sediment supply or groundwater level in structuring the braided geometry and the channel habitat pattern in natural braided rivers at a regional scale. We discussed the importance of local factors, such as the position of the groundwater table and the width of the active channel, for explaining the intensity of the braided pattern. There are numerous cases where the discharge is very low but the braided index high. These findings support management and conservation recommendations of braided rivers for implementing the European Water Framework Directive (2000).