Channel instability as a control on silting dynamics and vegetation patterns within perifluvial aquatic zones

Many authors have shown that the sedimentology of former channels and subse quent vegetation changes are controlled by temporal (flood events and succe ssional processes) and spatial (e.g. distance to the main channel) factors. River channel instability can disrupt these associations. The Ain River, F rance, has undergone a fluvial metamorphosis during the past 100 years, its braided pattern being replaced by a sinuous single-thread pattern. As a co nsequence, former channels have different geometrical characteristics and s ediment trap efficiencies. Former meandering channels experience more frequ ent backflows and are more rapidly silted than the older former braided cha nnels. The recently abandoned channels are characterized by the development of large-sized vegetation species with a relatively slow colonization rate , whereas the older channels are colonized predominantly by flood-tolerant aquatic plants. The locally derived discharge of former channels (from grou ndwater or from their own basin) may reduce or prevent sediment entry durin g flood events and thus may decrease the sedimentation rate. In such cases, the oligotrophic component of the water from the hillslope aquifer is high and the former channel is usually nutrient-poor, characterized by oligotro phic species. The main river channel also has experienced local incision, a ggradation and horizontal displacement during recent decades, so that the d ynamics of the former channels strongly depend on the dynamics of the reach in which they are located. In degraded reaches, former channels are often dry, and helophyte species have been replaced by mesophytes. The frequency and magnitude of flow connection between the river channel and the former c hannel can increase or decrease owing to the movement of the active river c hannel within the fluvial corridor, inducing varying modifications of forme r channel vegetation patterns. River channel instability at various time-sc ales is a key-factor controlling process diversity and thus biodiversity in the fluvial corridor. It can modify the geometry of abandoned channels, gr oundwater fluxes, the amount, mobilization and deposition of sediment withi n the corridor, and consequently the vegetation community patterns. This in creases the complexity of successional patterns, because an old former chan nel may be characterized by pioneer species whereas a younger one can becom e quickly filled and colonized by terrestrial species. Copyright (C) 2000 J ohn Wiley & Sons, Ltd.