Impact of Channel Bar Form and Grain Size Variability on Estimated Stranding Risk of Juvenile Brown Trout during Hydropeaking

Hydropeaking leads to artificial fluctuations in discharge and corresponding water levels with pronounced dewatering areas between base and peak flow along gravel bars and channel banks. In the present study, 16 hydropeaking reaches in Austria were investigated to assess possible differences in estimated stranding risk of young of the year brown trout (Salmo trutta) according to different gravel bar types and differences of microtopography roughness. Accompanied with grain size sampling along the various channel bars a conceptual stranding risk model (SRM) could be developed. The results showed that a high variability in estimated stranding risk exist for the tested sites considering discharge ratios of 1:3, 1:5 and 1:10. Moreover, the findings showed that especially point bar morphology was less sensitive to the risk of stranding compared to e.g. alternating gravel bars. Moreover, for benthic macroinvertebrates naturally formed backwater habitats (e.g. downstream of point bars) offer certain advantages over artificially created groin fields (e.g. 51 times more biomass). Considering the multiple pressures given for alpine rivers the improvement of structural features due to bar formation and related self forming processes was discussed as a possible alternative of future mitigation measures to reduce the negative impacts of hydropeaking.