Identification of rivers with high and good habitat quality: methodological approach and applications in Austria

The investigation of the 52 largest Austrian rivers with catchment areas > 500 km(2) (Danube River excluded) provides a national estimate of the ecological status of Austria's rivers and an example of the current status of European alpine rivers. Emphasis is placed on evaluation criteria, such as morphological character, instream structures, longitudinal river corridor, lateral connectivity and hydrological regime compared with original conditions. This assessment and evaluation of nearly 5000 river kilometers identifies the remaining river stretches with high habitat quality as well as those stretches that have been altered by systematic channelisation or hydropower development. Altogether, about 80% (3900 km) of the stretches in Austria's 'larger' rivers are moderately to heavily impacted by human activity and thus no longer correspond to their original channel form and dynamics. Impoundment, water diversion and hydropeaking each impact the hydrology of approximately 20% of the above stretches. Only about 20% of all investigated river stretches can still be classified as having high or good habitat quality and as retaining essentially natural functions. Using natural conditions as a reference, the most severe losses have occurred in reaches that were originally braided or meandering. Analysing the amount of unaltered river stretches according to biocoenotic regions, 37% (523 km) of the originally rhithral (trout) region (1416 km) and 18% (480 km) of the originally hyporhithral (grayling) region (2739 km) remain today in high or good habitat quality. Only a few intact river stretches (totaling 43 km) can be found along formerly potamal river reaches (731 km). These data underline the rarity and, therefore, national importance of the remaining unspoiled aquatic habitats. The results presented here form a basis for water resource management and nature conservation strategies, as well as for restoration programs. In particular, these investigations help to fulfill the requirements of the EU-Water Framework Directive by assisting to delineate and characterise reference sites of different river types.