Overbank sedimentation rates in former channel lakes : characterization and control factors
Résumé
Overbank sedimentation rates were studied in former channels of three rivers in south-eastern France. Depth and spatial distribution of sediment, as well as geometry, hydrological connectivity and age of 39 lakes, were both measured and calculated. The mean sedimentation rate of lakes varied between 0 and 2·57 cm year−1. Sedimentation rates are linked to water depth and often undergo a decreasing gradient from the downstream outlet to the inner part of the lake. Multiple regression modelling demonstrates that sediment depth is essentially a function of overbank flow frequency. The greater the difference between upstream and downstream overbank flow frequency, the faster the sedimentation rate. These differences in sedimentation rates also correspond to different former channel geometry: the rates are slower in narrow and straight channels (former braided, and point-bar backwater channels), and faster in large and sinuous channels (exhibiting meanders, anastomosing channels and coves). The suspended sediment flux is variable from one reach to another, the middle reach of the Rhône conveying more sediment than the upper reaches, the Doubs or the Ain reaches. The suspended sediment flux does not explain a statistical difference in lake sedimentation rates between the reaches, which also provide clear evidence of the importance of local connectivity controls. Sedimentation patterns were also complicated by temporal changes in lake connectivity associated with geomorphological or anthropogenic changes operating within the main channel.
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