Census and typology of braided rivers in the French Alps |
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| Authors: | Hervé Piégay Adrien Alber Louise Slater Laurent Bourdin |
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| Institution: | 1. University of Lyon, CNRS-UMR 5600, Site Ens-lsh, 15 Parvis R. Descartes, F-69342, Lyon, France
2. Agence de l’Eau Rh?ne-Méditerranée-Corse, Lyon, France
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| Abstract: | Following the implementation of the European Water Framework Directive (WFD) and the need to reach a “good ecological status”
for rivers, key-questions are being raised about braided rivers. Before any environmental policy can be drawn up, these rivers
need to be located, long term changes must be evaluated, and the regional diversity of such systems must be understood, as
their inner complexity has not yet been well studied. Therefore, the aim of this work is to carry out a census of the braided
channels of the French Alps and to establish a typology based on basic geomorphic indicators. A minimum estimate of the cumulative
length of braided rivers prior to major infrastructure construction amounted to 1214 km. Around 53% of these rivers have disappeared
during the last two centuries in relation to embankment or channelization, but a loss of 17% is still unexplained. The range
in catchment size, mean slope and active channel width has been determined for the Western Alpine braided channels as well
as the range in changes due to narrowing, widening and shifting. Seven types of braided rivers have been distinguished based
on geographical settings (climate conditions and geology) and differences in terms of adjustment to human pressure on peak
flow and sediment delivery. The percentage area of islands in the active channel and the relative length of banks also show
a regional difference. Maximum and minimum thresholds of braided activity have been established taking into account the active
channel width and the catchment area. The position of the studied reaches between these two thresholds are discussed in relation
to position of rivers known in the literature, considering both long-term trends and short-term fluctuations in channel width. |
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