What is the Relative Effect of Space and Time to Explain the Braided River Width and Island Patterns at a Regional Scale?

Abstract

Several decades of human activities have severely impacted braided rivers worldwide. Despite their widespread disappearance, some remnant braided sectors are still held in the French Rhone basin, mainly in the south-east of France. In this paper, we analyse the evolutionary pattern of 53 braided reaches, focusing on the active channel width and island patterns, by comparing aerial photographs from the 1950s and 2000s (Institut Géographique National). Because different braided patterns exist (e.g. bar versus island-braided), we tested the relative effect of geographical and temporal factors. The hypothesis is that three main biogeomorphological braided types exist (i.e. defined through the presence, the amount and the relative size of vegetated islands), based on the combined effect of the following: (i) their position along the river network (i.e. river gradient, altitude and sediment regime) and (ii) the temporal effect represented by the time since the last large flood, that is, the recent flood history. Our results show that even if the regional context (climate and sediment regime mainly) plays a key role, the temporal factor, represented by recent flood history, seems to heavily influence the response of the width pattern and vegetation recovery. Local factors (i.e. topography and groundwater) may also have an impact, but their influence has no effect at the regional level. These results support braided river management (conservation and/or restoration actions) in the Rhone basin and provide a better understanding of the range of braided rivers' functioning. Further studies (e.g. multidate retrospective survey) are needed to better understand the role of flood events on braided pattern and vegetation recovery. Copyright © 2013 John Wiley & Sons, Ltd.