Revitalisierungsmaßnahmen für Augewässer - Möglichkeiten und Grenzen
The key factors for understanding the ecology of large rivers and their adjacent floodplains are extent, duration and dynamics of the hydrological connectivity between the two compartments both via surface water and groundwater. Disturbances by floods are essential. They initiate successions and foster biodiversity by enabling weak competitors to coexist. Disturbances produce a dynamic equilibrium of erosion and sedimentation processes, which in turn provide the basis for a high habitat diversity. The hydrological connectivity between river and floodplain determines the nutrient and sediment exchange processes as well as the development of macrophytic vegetation in the backwaters. Connectivity is also a decisive factor for several rheophilic species of fish which use backwater areas as refugia or feeding zones during certain phases in their life history. This hydrological connectivity has been strongly reduced in all the larger European rivers due to regulation and damming. The diminished hydrological exchange and dynamics resulted in terrestrification processes in the backwater zones, a clogging of the groundwater aquifer and thus a reduction of exchange conditions between river and floodplains. A decline of highly adapted floodplain species as well as a loss in limnological functions is the consequence. In order to restore functions, concepts are currently being developed and tested to improve the water budget of backwaters and their connectivity with the river. Planning such concepts and implementing such programme requires that the present-day conditions of the aquatic and semiaquatic biotopes and their developmental trends be analysed. The present paper discusses these aspects using the restoration programmes developed for the Austrian Danube as an example.