Bertrand Moulin

Spatial and temporal variability in sedimentation rates associated with cutoff channel infill deposits: Ain River, France

Floodplain development is associated with lateral accretion along stable channel geometry. Along shifting rivers, the floodplain sedimentation is more complex because of changes in channel position but also cutoff channel presence, which exhibit specific overflow patterns. In this contribution, the spatial and temporal variability of sedimentation rates in cutoff channel infill deposits is related to channel changes of a shifting gravel bed river (Ain River, France). The sedimentation rates estimated from dendrogeomorphic analysis are compared between and within 14 cutoff channel infills. Detailed analyses along a single channel infill are performed to assess changes in the sedimentation rates through time by analyzing activity profiles of the fallout radionuclides 137Cs and unsupported 210Pb. Sedimentation rates are also compared within the channel infills with rates in other plots located in the adjacent floodplain. Sedimentation rates range between 0.65 and 2.4 cm a1 over a period of 10 to 40 years. The data provide additional information on the role of distance from the bank, overbank flow frequency, and channel geometry in controlling the sedimentation rate. Channel infills, lower than adjacent floodplains, exhibit higher sedimentation rates and convey overbank sediment farther away within the floodplain. Additionally, channel degradation, aggradation, and bank erosion, which reduce or increase the distance between the main channel and the cutoff channel aquatic zone, affect local overbank flow magnitude and frequency and therefore sedimentation rates, thereby creating a complex mosaic of sedimentation zones within the floodplain and along the cutoff channel infills. Last, the dendrogeomorphic and 137Cs approaches are cross validated for estimating the sedimentation rate within a channel infill

Wood entrance, deposition, transfer and effects on fluvial forms and processes: Problem statements and challenging issues

Throughout the world before the 1970s, in-channel large wood (hereafter LW), was generally considered a nuisance or a hazard to be avoided because of the hydraulic effects and consequential associated risks that it could produce. LW was systematically removed from channels for a perceived benefit to human activities. Over the last four decades, LW has received an increasing interest among scientists who recognise it as a significant structural and functional component of aquatic ecosystems. Research in geomorphology has addressed LW characters and its effects on flow hydraulics, on channel and valley forms and their evolution, on the storage and transfer of sediment and organic matter, as well as on associated habitats for aquatic communities. After considering the space-time framework within which the wood dynamics can be studied, we highlight the LW effects on channel morphology and fluvial processes, the ecological and societal consequences of wood in rivers and implications in terms of river management and restoration. After centuries of wood removal, LW is now introduced in rivers for ecological improvement in some areas of North America, Europe, Australia, and Japan.