Ecological rehabilitation of the lowland basin of the river Rhine (NW Europe)

In this paper, the status of ecological rehabilitation of the Dutch lowland basin of the river Rhine has been reviewed. The historical perspective, mainly with regard to river regulation measures in the past, is given. The lower river Rhine comprises a man-dominated, strongly regulated catchment, polluted water and sediments, and annihilated and deteriorated ecosystems. During the past 25 years, the water quality and, to a lesser extent, the sediment quality, has improved considerably, hence leading to improvement of biotic diversity. The rehabilitation of lost and disturbed ecosystems started some 15 years ago. The non-refutable boundary conditions for restoration projects are protection against flooding and transport by cargo ships. Holistic ecological theories to promote river catchment management are available, but the rehabilitation projects dominantly rely on local and regional planning policies. The use and application of ecological criteria are increasingly playing a role in rehabilitation schemes. Ecological rehabilitation mainly focuses on nature conservation and restoration strategies, i.e. exploiting the hydrodynamic and morphodynamic potentials of the flowing river, and introducing a semi-natural grazing regime by large herbivores. Particularly the creation of new secondary channels contributes to the restoration of riverine habitat diversity and heterogeneity, and hence to the species diversity. The assessment of a number of rehabilitation projects has led to the conclusion that no general statement can be given concerning 'success' or 'failure' of specific restoration measures. The overall balance of 14 assessed projects out of approximately 30 running projects is positive. The highly dynamic river Waal offers the best possibilities for the restoration of natural sandy levees and back swamps. It will be very difficult to rehabilitate the hardwood floodplain forests under the present management conditions. Further recovery of the river biota depends on continued decrease of pollution, an increase of morphological dynamics, and the development of the original natural habitats.