Decadal changes in distribution and frequency of wood in a free meandering river, the Ain River, France

Abstract : This study examined the temporal dynamics and longitudinal distribution of wood over a multi-decadal timescale at the river reach scale (36 km) and a meander bend scale (300? 600 m) in the Ain River, a large gravel-bed river flowing through a forested corridor, and adjusting to regulation and floodplain land-use change. At the 36 km scale, more wood was recruited by bank erosion in 1991?2000 than since the 1950s. The longitudinal distribution of accumulations was similar between 1989 and 1999, but in both years individual pieces occurred homogeneously throughout the reach, while jam distribution was localized, associated with large concave banks. A relationship between the mean number of pieces and the volume recruited by bank erosion (r 2 = 0·97) indicated a spatial relationship between areas of wood production and storage. Wood mass stored and produced and channel sinuosity increased from 1993 to 2004 at three meander bends. Sinuosity was related to wood mass recruited by bank erosion during the previous decade (r 2 = 0·73) and both of these parameters were correlated to the mean mass of wood/plot (r 2 = 0·98 and 0·69 respectively), appearing to control wood storage and delivery at the bend scale. This suggests a local origin of wood stored in channel, not input from upstream trapped by preferential sites. The increase in wood since 1950 is a response to floodplain afforestation, to a change from braided to meandering channel pattern in response to regulation, and to recent large floods. We observed temporal stability of supply and depositional sectors over a decade (on a reach scale). Meander bends were major storage sites, trapping wood with concave banks, also delivering wood. These results, and the link between sinuosity and wood frequency, establish geomorphology as a dominant wood storage and recruitment control in large gravel-bed rivers.
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Submitted on : Friday, May 16, 2008 - 11:28:50 AM
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Neil Lassettre, Hervé Piégay, Simon Dufour, Anne-Julia Rollet. Decadal changes in distribution and frequency of wood in a free meandering river, the Ain River, France. Earth Surface Processes and Landforms, Wiley, 2007, 32, pp.1098-1112. ⟨10.1002/esp.1605⟩. ⟨halshs-00280059⟩

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