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P. Davy, J. Lavé, and L. , Sklar were greatly appreciated the Editor, and two anonymous reviewers have helped improving the paper. This work has been funded by a CNRS/INSU/RELIEFS project, Dynamique des réseaux hydrographiques " and supported by the French Agence Nationale de la Recherche (ANR) under grant ANR?05?blan?0273

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