Changing river channels: The roles of hydrological processes, plants and pioneer fluvial landforms in humid temperate, mixed load, gravel bed rivers Angela M. Gurnell a, ⁎, Walter Bertoldi a, b , Dov Corenblit c a Queen Mary, University of London, School of Geography, London E1 4NS, United Kingdom b University of Trento, Department of Civil and Environmental Engineering, Via Mesiano 77, 38123 Trento, Italy c CNRS, UMR 6042, GEOLAB, Laboratoire de Géographie Physique et Environnementale, F-63057 Clermont-Ferrand, France abstract article info Article history: Received 28 May 2011 Accepted 19 November 2011 Available online 1 December 2011 Keywords: Fluvial processes Riparian vegetation Aquatic vegetation Ecosystem engineer Pioneer landform River morphodynamics The fluvial riparian and aquatic patch mosaic varies along rivers according to geomorphological setting, hy- drological 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 develop- ment 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 pro- cesses, we review research evaluating characteristic vegetation-mediated landform development. We inves- tigate 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 pre- sent 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 engi- neering 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 corri- dor as river energy and vegetation colonisation and growth change. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In 1984, Hickin commented that ‘the physical science of fluvial geomorphology is flawed because it ignores processes that are not easily quantifiable and physically or statistically manipulable. The in- fluence of vegetation on river behaviour and fluvial geomorphology is a set of these processes' (p111). Since the publication of Hickin's paper, research on interactions between vegetation and fluvial geo- morphology has developed rapidly. Very significant progress has been made in not only recognising vegetation as an extremely impor- tant geomorphological agent but also in understanding the environ- mental circumstances and the mechanisms by which fluvial systems are moderated, even controlled, by vegetation. The structure and function of riparian and aquatic vegetation vary along river systems with geomorphological setting, hydrological re- gime, sediment supply and surface–groundwater connectivity (Tabacchi et al., 1998; Robertson and Augspurger, 1999; Nakamura et al., 2000). This reflects the fact that plants colonising riparian and aquatic zones have to cope with an environment where flood Earth-Science Reviews 111 (2012) 129–141 ⁎ Corresponding author. Tel.: +44 2078828927; fax: +44 20 7882 7479. E-mail addresses: a.m.gurnell@qmul.ac.uk (A.M. Gurnell), walter.bertoldi@ing.unitn.it (W. Bertoldi), dov.corenblit@univ-bpclermont.fr (D. Corenblit). 0012-8252/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2011.11.005 Contents lists available at SciVerse ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev