German politicians have promised that the River Rhine will be sufficiently restored within twelve years to permit salmon to live there again. Obviously the large rivers in Central Europe are more isolated from each other than smaller streams, and communities donating potential colonizers (if they exist at all) are further apart for possibly restored large rivers than for smaller streams. Thus, recovery can be expected to be faster in small streams than in big rivers after restoration (or reduction of detrimental human influence). Therefore, two restoration projects in German lowland streams, which differ in their degree of isolation, can serve as an indicator to the time periods which could at least be expected for the recovery of Central European rivers.
Under optimal conditions (almost completely intact communities upstream and downstream of a 400 m restored reach) in North Germany, sufficient recovery of benthic macroinvertebrate fauna could be achieved in relatively short periods. However, in a rather isolated stream reach in the Upper Rhine valley (closest intact lotic ecosystems of a comparable type were found 20-25 km away) a sufficient recovery of benthic macroinvertebrate fauna was not achieved within five years after restoration, although there was high diversity of physical habitats and the water quality was acceptable (except for two oil accidents in the fourth and the fifth year).
Hence, we conclude that recovery of a large Central European river ecosystem like the Rhine, which has lost a large number of its former species and is more isolated than small streams, will require more than twelve years to reach a state significantly different from the present one.