Changing river channels: the roles of hydrological processes, plants and pioneer fluvial landforms in humid temperate, mixed load, gravel bed rivers.

International audienceThe fluvial riparian and aquatic patch mosaic varies along rivers according to geomorphological setting, hydrological regime, sediment supply and surface–groundwater connectivity. This relation between physical processes and plants is not unidirectional. Once established, riparian and aquatic plants frequently act as physical ecosystem engineers by trapping and stabilising sediments, organic matter and the propagules of other plant species, modifying the local sedimentary and morphological environment by driving the development of landforms and associated habitats, and so facilitating the rapid establishment of other plants that can in turn reinforce the development of landforms such as river banks, vegetated islands and floodplains. This paper reviews knowledge on the hydrogeomorphological significance of riparian and aquatic vegetation with a particular emphasis on humid temperate, mixed load, gravel bed, floodplain rivers.First, we investigate how vegetation dynamics across river margins are governed by hydrological processes that can both promote riparian vegetation growth and disturb and destroy riparian and aquatic vegetation. We show, with some simple numerical modeling, that different combinations of moisture supply and flow disturbance have the potential to generate many different responses in the lateral distribution of vegetation biomass along river corridors.Second, building on the varied lateral biomass distributions that are primarily dictated by hydrological processes, we review research evaluating characteristic vegetation-mediated landform development. We investigate aquatic and riparian plants acting as physical ecosystem engineers by creating and modifying habitats in river systems with sufficient suspended sediment supply for habitat or landform building. These plants have a crucial impact on sediment stabilisation and pioneer landform building along the interface between plant (resistance) dominated and fluvial-disturbance (force) dominated zones of the river corridor. We present some examples of vegetation-mediated landforms along rivers with strongly contrasting hydrological regimes and thus lateral distributions of vegetation.Lastly, we present a conceptual synthetic model that links the development of pioneer landforms by engineering plants with river morphology and morphodynamics in humid temperate, mixed load, gravel bed, floodplain rivers. Drawing on four example rivers, we show how different plants and pioneer landforms act at the interface between the plant dominated and fluvial-disturbance dominated zones of the river corridor as river energy and vegetation colonisation and growth change