The restoration of a 1.3 km straightened and channelized reach of the River Gelså to a new 1.9 km meandering course is one of the largest river restoration projects so far undertaken in Denmark. In order to elucidate the ecological impact of restoration, the macroinvertebrate community present in the reach prior to and following its restoration was compared with that in an upstream reach that remained straight and channelized.
Two years after restoration of a meandering course, macroinvertebrate density and diversity was greater than in the upstream control reach, Gammarus pulex L. being particularly abundant in the restored reach. In general, species preferring a stony habitat seemed to favour the new reach; Heptagenia sulphurea Müll. was only found on stones in the restored reach, and was totally absent from the control reach. Two other stone-preferring species, Ancylus fluviatilis Müller and Hydropsyche pellucidula Curtis, were found in higher density in the control reach; however, this probably reflects competition for the limited space available on stones.
The study shows that river restoration can have a positive impact on macroinvertebrate community structure, and probably higher trophic levels also, such as fish, due to increased numbers of potential prey organisms. In addition, the study suggests that the impact of restoration projects on the macroinvertebrate community is best understood by undertaking both qualitative and quantitative studies.