An evaluation of the benefits of lowland river-floodplain rehabilitation (the Rhine, the Netherlands).

Abstract

Water quality of the Rhine has improved significantly since 1970, but its landscape was highly altered with a shortage of dynamic processes, characteristic landscape elements and biodiversity. During the last decade manifold floodplain rehabilitation projects have been planned and realized in the Netherlands. Yardsticks for recovery are naturalness, biodiversity and spatial connectivity, but actually there is no clear end-point or reference to assess its success. Constraints for rehabilitation are flood protection and shipping. The benefits of a subset of rehabilitation projects situated along the largest flee-flowing Rhine branch, that has an open connection to the North Sea, were assessed at two landscape levels: the project itself (1-10 kin) and the river stretch (> 100 km) using five parameters (landscape mosaic, vegetation, macro-invertebrates, fish and birds). The assessment yielded comparable marks among the parameters, so the overall appraisal is considered to be robust. However, the scarcity and incompleteness of the underlying data forced us to use a fair amount of expert judgement. Generally terrestrialized floodplains have been set back to more frequently flooded (semi-) aquatic habitats. Especially dynamic, shallow water habitats emerged. Vegetation quickly responded when management changed from farming to extensive grazing. Rheophilic fish and macro-invertebrates immediately colonised side channels. At present, more than 40 % of the present floodplain area consists fo deep water and 30 % of agricultural land and built up areas, and typical floodplain habitats (such as shallow riverbed, river banks and dunes and herbaceous floodplains) are still underrepresented. Within rehabilitation projects the landscape significantly changed, but the projects do not yet significantly contribute to the ecological recovery of the whole river stretch. Moreover, biological indicators point to a moderate or even poor ecological state.