River restoration: separating myths from reality

Abstract:

River restorations are a social construct where degraded systems are physically modified to obtain a pre-disturbance set of attributes. These can be purely esthetic but are often linked to some kind of biotic recovery or the provision of important ecosystem services such as flood control or self-purification. The social setting of restoration projects, with a range of potential conflicts, significantly reduces scale of most interventions to a size with little room, or wish, for natural processes. We show that projects sizes are still very small and that the restoration target is not to recover natural geomorphic processes but rather to fulfil human perception of what a nice stream looks like. One case from Danish lowland streams, using a space-for-time substitution approach, shows excess use of pebble and gravel when restoring channelized sandy bottom streams, de-coupling the link between energy and substrate characteristics that are found in natural lowland systems. This has implication for both the biological structure and functioning of these systems as a direct link between substrate heterogeneity and macroinvertebrate diversity was not found in restored streams, while the density of grazer increased indicating an increased use of periphyton as a basal resource. Another case of adding woody debris to UK lowland streams, using a BACI study design, showed very little effect on the macroinvertebrate community even after a 100-year flood, which indicate that added tree trunks did not provide additional flow refugia. We suggest that restoration schemes should aim at restoring the natural physical structural complexity in the streams and at the same time enhance the possibility of re-generating the natural geomorphological processes sustaining the habitats in streams and rivers.