Abstract

The Allier is a steeply sloping, rather small gravel-bed river. The lower Volga has a large discharge, a sand bed and a small valley gradient. In both studied river reaches point bars are superimposed by scroll bars and chute bars. In an absolute sense the rate of morphodynamic changes in both rivers is about the same. When scaled to the size of the rivers, however, morphological processes in the Allier are operating one order of magnitude faster. In point bars of wide bends in the Allier chute bars and scroll bars are found respectively in the upstream and downstream part of the point bar; in tight bends chute bars migrate further downstream and coalesce with scroll bars. The development on a point bar of a vegetation cover and related reduction in flow velocity by increased hydraulic roughness showed to have opposite effects on chute bar activity: In the Allier the chute bars became inactive. In the Volga study area, however, flow blockage by vegetation redirected the flow into a chute, resulting in an expansion of the chute bar. A third type of bar emerging at low river stage is formed by accumulations behind trees, bushes or large wooden debris. As the existing term Large Wooden Debris islands is not appropriate, for these typically tail-shaped bars the term tail bar is proposed.

Keywords: scroll bar, chute bar, river morphology, vegetation