DISTRIBUTION OF LARGE WOODY DEBRIS ALONG THE OUTER BEND OF MEANDERS IN THE AIN RIVER, FRANCE

Abstract

The distribution of large woody debris (LWD) was studied along the concave outer bend of three meanders in the Ain River, a 195-km-long tributary of the upper Rhóne River. The Ain River is a sixth-order channel dominated by a gravel-cobble bed substrate that meanders through a floodplain covered largely by riparian forest vegetation. The mass of LWD was measured in a 15-m-wide forest band along the three meander bends, with total loads calculated to be 56.1 t ha^-1 at the Mollon study site, compared with 22.9 and 21.5 t ha^-1 at the study sites of Bublane and Blyes, respectively. The distribution of LWD within any one meander concavity was dependent on three main variables: (1) the position of the concavity in relation to the main flow axis, (2) the height of the bank, and (3) the presence and position of overbank flow channels in the concavity. The type of vegetation community along the channel margin is nondiscriminating, favoring the conclusion that the LWD comes mainly from upstream of the bends rather than locally. The relative influence of each variable is contrasted between the three study sites. The total LWD deposited along concavities was most strongly controlled by meander-bend geometry. The supply areas for LWD, located a few kilometers upstream from the study sites, were also found to influence total LWD along concavities. Findings from this study are applicable to managing instream large woody debris as part of ongoing efforts to restore alluvial forests along French rivers. [Key words: large woody debris, debris line, meandering piedmont river, Ain River, France.]