The Wullowitz-Lakes theory: Periglacial impacts on river morphology and habitat suitability for the endangered freshwater pearl mussel

The Malše River, on the border between Austria and the Czech Republic, contains one of the last remaining freshwater pearl mussel (Margaritifera margaritifera) populations on the Bohemian Massif. Evidence from field studies indicated that landscape evolutionary processes changed the boundary conditions for river channel formations, especially in those parts where the remaining mussels, probably also self-reproducing to a limited extent, were documented. The aim of the present work was to study these possible palaeo-impacts through (i) the analysis of the terrain data and accompanying hydrodynamic-numerical modelling and (ii) sediment sampling of the main channel and the surrounding floodplains at the reach and catchment scales. The results indicated that solifluction caused a historic blockage in the drainage area with consequent formation of lakes in the upstream part of these obstructions. At least two sites where this “Wullowitz-Lakes Theory” can be applied were identified. The occurrence of these lakes was validated by sediment analysis. The findings further indicated that the later fluvial processes in these deposits of a former stagnant water body forced the formation of an anabranching channel in the system, which would not have occurred in a solely freely developed fluvial system. In conclusion, it could be determined that the consideration of palaeo-processes might be important even for present onsite assessments of aquatic habitats and species such as the endangered freshwater pearl mussel.