Altered hydrology and sediment budget

In the section under consideration, the discharge of the Traun is relatively low. The diversion of the Traun at the Traun weir of the Kleinmünchen power plant (see photo) creates an almost six-kilometre-long residual flow section. In this area, the discharge of the Traun is lower than its natural discharge due to the use of the power plant. At the same time, the backwater of the Danube power plant Abwinden-Asten extends into the area where the Traun flows into the Danube. This creates an untypical flow character, which leads to strongly changing conditions for aquatic organisms. In addition, the bedload and sediment balance in the Traun is strongly altered. Particularly in the dammed area upstream of the power plants and in the residual flow section, riverbed incision occurs. The result is a loss of flowing water character, deterioration of the habitats for aquatic organisms, but also a drop in the groundwater level, which leads to decoupling and the drying out of the adjacent floodplains.

Water temperature

For several decades, the average water temperature on the Traun has been rising steadily. This trend is basically due to climate change, but is amplified by other factors (such as discharges into the Traun, river regulation or lack of shading). In the considered section of the Traun, the low discharge contributes significantly to warming. In particular, species that depend on low water temperatures are constantly losing their habitat as a result.

Fish population

The three fish species Grayling (shown in the photo and recognisable by the distinctive dorsal fin), Barbel and Nase, which are specific to the Traun, were detected in all existing surveys. Associated species that prefer higher flow velocities, such as Brown trout, European Chub, minnow, gudgeon, hazel, bullhead and loach, were also detected in almost every fish monitoring. Accompanying species that prefer calmer water areas with higher turbidity, such as bleak, roach, ide or pike, on the other hand, were hardly or not at all detected. The demanding mid-distance migrants huchen and burbot were also only recorded with a single specimen (huchen) or not at all (burbot).

As the River Development and Risk Management Concept for the Traun only covers a relatively  short section, the concept for measures is very detailed. The figure provides an insight into the concept of measures, which at the same time forms the basis for the pilot measure and already envisages very concrete changes to the river course of the Traun.