Hydrogeomorphic processes affecting riparian habitat within alluvial channel-floodplain river systems: a review for the temperate zone

J. Steiger, E. Tabacchi, S. Dufour, D. Corenblit, J.-L. Peiry
2005 Rivers Research and Applications: an international journal devoted to river research and management  
Hydrogeomorphic processes within alluvial river systems create, maintain and degrade riparian habitat. The dynamic interactions between water, sediment, aquatic-terrestrial landforms and biotic elements control the functional processes and biodiversity patterns within the riparian zone and, thus, contribute directly to their ecological integrity and societal value. Numerous researchers from different disciplines publish work on the physical, biological, economic and societal functions of the
more » ... arian zone within various physiographic areas. The present paper aims to review the hydrogeomorphic processes of unconfined alluvial channel-floodplain rivers within the temperate zone. These processes and their interactions with the biotic environment provide the basis for understanding the physical as well as the ecological functioning of fluvial hydrosystems. The review focuses mainly on the European context, but major advances in riparian research from other continents are also considered. Rehabilitation and management strategies for the riparian zone are summarized and recommendations for further research conclude this review. The riparian zone can be viewed as a four-dimensional sub-system of fluvial hydrosystems (sensu Amoros et al., 1987; Petts and Amoros, 1996). It may be several tens of kilometres wide, as in the Orinoco and Amazon River basins (Rosales-Godoy et al., 1999) or a narrow strip of streambank vegetation as in canyons and V-shaped valleys (Malanson, 1993) or in arid and semi-arid regions (Salinas et al., 2000) . Two main conceptual frameworks delimiting the riparian zone co-exist with intermediate positions according to the authors' conception and the geographical region studied. The first framework considers the riparian zone in a more spatially restrictive sense of within bank and around bankfull discharge (e.g. Hupp and Osterkamp, 1996) . The second, which views the riparian zone in a broader sense, includes the ecosystem adjacent to the river channel (Malanson, 1993) which may include the entire floodplain, and, according to Stanford et al. (1996) also terraces, i.e. former floodplains. In any case, Malanson (1993) points out that the use of the term floodplain as a synonym for riparian zone would be misleading because the riparian zone also includes narrow strips along downcutting rivers, islands, and channel landforms. In unmanaged fluvial systems, the riparian zone constitutes highly dynamic areas within the fluvial landscape. The banks of alluvial rivers are less permanent than most other aspects of the landscape (Schumm and Winkley, 1994); and highly dynamic tributary junctions are also recognized to play a major role in providing riparian habitat diversity (Benda et al., 2004) . However, under increased human pressure, the riparian zone is coveted by numerous and often conflicting activities (e.g. agriculture, fishing, tourism, urbanization, sediment mining), leading often to significant modification of riparian zone structure and functioning by riverine societies. Stream structure, habitat and function can be influenced by patch characteristics within fluvial systems which in turn determine biotic and abiotic processes at different spatio-temporal scales (Pringle et al., 1988) . Patch characteristics also reflect the dynamic mosaic of resource and disturbance patches of different age and successional stages (Petts, 1990a) . A disturbance occurs when potentially damaging forces (e.g. high-flow or low-flow events) are applied to a habitat space occupied by a population, community, or ecosystem (Lake, 2000). Lateral instability, avulsion and variations in hydrogeomorphic conditions in temperate fluvial systems create patches of fluvial landforms that normally exert a profound influence on the vegetation patterns (Hupp and Bornette, 2003) and lead to a particularly diverse range of habitats at different spatial and temporal scales. Thus, hydrogeomorphic processes affect aquatic and riparian ecosystems through their influence on physical and biotic habitat structure, although biological processes can, in turn, influence physical processes (Montgomery, 1999) . The range of riparian habitats includes localized micro-habitats formed of sediment, rocks and dead wood; meso-habitats formed of geomorphic landforms (e.g. bars, natural levees, sloughs, side and abandoned channels) and riverine vegetation; and macro-habitats formed of riparian forests and entire reaches and sections of the river corridor. The ratio between each habitat scale and the river section observes changes according to stream order from headwater streams to the river mouth and also depends on the river style (e.g. braided, anastomosed, meandering). Richards et al. (2002) suggest that the reach scale is the most intimate with mutual association between channel and vegetation (patch) dynamics, and the greatest potential for biogeomorphological management ( Figure 2) . Within floodplain rivers, riparian habitat quality is determined by mineral substrate, aquatic plants, riparian forests, water depth (instream and overbank) and current velocity (lentic or lotic), and habitat diversity (e.g. islands, cutoffs, benches). Riparian habitat functions as a food source, refugia (e.g. shelter from fast currents, hiding from predators) and reproduction site (e.g. spawning, nursery) for aquatic, amphibian and terrestrial organisms and communities. It plays a key role in providing refugia for recovery from natural (e.g. floods, droughts) and anthropogenic (e.g. accidental pollution) disturbances in river systems. Refugia are defined as habitats or environmental factors that convey spatial and temporal resistance and/or resilience to biotic communities impacted by biophysical disturbances (Sedell et al., 1990). According to these authors most refugia in rivers are characterized by extensive coupling of the main channel with adjacent streamside forests, floodplain features and groundwater. Ward et al. (1998) note that the generally high resilience of lotic ecosystems to disturbance is attributable, in part, to high spatio-temporal heterogeneity; and also because habitat patches less affected by a particular perturbation may serve as refugia from which survivors may recolonize more severely affected areas. The riparian zone does not necessarily have to be characterized by woodlands. Poplar plantations, agriculture, or spontaneous grass and shrubby vegetation also occupy the riparian zone. Geomorphic (e.g. rocky substrate) or climatic (e.g. semiarid regions) conditions may also constrain the theoretically older woody successional stages to herbaceous or shrubby stands. However, at a certain successional stage mature riparian vegetation will establish HYDROGEOMORPHIC PROCESSES AFFECTING RIPARIAN HABITAT 721
doi:10.1002/rra.879 fatcat:tfr5zf6ozndpdiv3hhxy3vviau