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Angela M. Gurnell Department of Geography, King’s College London, Strand, London WC2R 2LS Abstract: The characteristics of river channels and their margins are primarily driven by the river’s flow regime and the erosion, transport and deposition of sediment. However, interactions between fluvial processes and riparian vegetation are increasingly being recognised as important influences on the geomorphology of the active zone of rivers. This paper explores interactions between riparian plants, particularly trees, and river geomorphology. It illustrates how such interactions occur in relation to a range of processes and across different spatial scales. Riparian vegetation fragments and propagules form part of the sediment load of rivers and so analogies can be drawn with the transfer of mineral sediment within river systems. Whole plants, vegetation fragments and propagules are delivered to the river by physical processes but the the timing of seed release and the morphological characteristics of the plants moderate delivery. The transport and deposition of plant material is partly controlled by flow patterns but is also influenced by the buoyancy and morphology of the material. Once deposited, the vegetation fragments, propagules and the plants that develop from them can have significant local effects on flow hydraulics and sediment erodibility, producing complex assemblages of physical features. Thus riparian vegetation interacts with fluvial processes in many ways. The geomorphological significance of interactions between riparian vegetation and fluvial processes is illustrated at a range of spatial scales (from small patches to the entire length of the river) with reference to the Tagliamento River, Italy. Here ribbons of transported fine sediment, vegetation fragments and propagules are retained as a result of geomorphological filtering within the river corridor, with vegetation influencing both the transfer and retention processes. For example, at the patch scale individual uprooted trees deposited on exposed riverine sediments create local hydraulic complexity that induces the scour and deposition of sediment and propagules, resulting in a suite of distinctive landforms which support a diverse vegetation community. At the scale of the entire river continuum, downstream changes in the dominant riparian tree species influence the characteristics of the plant materials delivered to the river and thus the nature of the vegetation-geomorphology interactions that are feasible or which dominate at the patch scale. Within individual sectors or reaches, interactions between riparian vegetation, mineral sediment, stream power and the hydrology of alluvial gravels are reflected in the transfer or retention of fine sediment and plant materials, the rate of vegetation growth from retained propagules, and the integration of patch-scale effects that supports the spatial pattern and dynamics of riparian margins including islands. |
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