HYDROLOGICAL CONNECTIVITY AND FLOOD PULSES AS THE CENTRAL ASPECTS FORTHE INTEGRITY OF A RIVER-FLOODPLAIN SYSTEM

Preparations are currently under way to establish a river restoration concept for the free-flowing section of the Austrian Danube downstream of Vienna. This should serve as a basis for a National Park managemen t. Studies have been initiated to assess the effects of hydrological c onnectivity, flood pulses and successional processes on backwaters wit h regard to their hydrochemistry and suspended solid load. Water level fluctuations in the ground- and surface waters of the floodplain are dynamic because of the porous aquifer structure and several inflow are as. In a lateral transect, increasing distance from the Danube correla tes with decreasing connectivity. Different influences in a longitudin al transect are due to the positions of the inflow areas. At low water level, the Danube and its backwaters have no surface connection. Inte rnal processes determine the conditions within the backwaters, which a re typically characterized by low concentrations of inorganic nutrient s and particles versus the Danube. At higher water levels, local surfa ce inflow and seepage water lead to periodic nutrient pulses and eutro phication. The input of inorganic nutrients and particles, as well as the flushing of phytoplankton during floods, establish an initial setp oint in the backwaters. The significance of flood pulses for the dynam ics of biological processes is emphasized by short-term changes, e.g. in the relationship between nutrients, inorganic particles and chlorop hyll. The response in different water bodies depends on the grade of c onnectivity to the Danube.