An increasing number of rivers have been restored over the past few decades but only a small number of these projects have been monitored, and hence, the knowledge on the effect of river restoration on biota is limited. Nevertheless, monitoring results of several projects are available in peer-reviewed scientific literature and have been compiled in recent research projects. The objective was to evaluate the effect of hydromorphological restoration on biota based on these existing data. Specific objectives were to quantify restoration success, to identify catchment, river reach, and project characteristics which influence (either constrain or enhance) the effect of restoration, and to derive recommendations for river management. The study is complemented by a satellite topic on urban river restoration.

Some narrative reviews have already been published, but a comprehensive quantitative meta-analysis which summarizes the findings of these existing studies was lacking. In the meta-analysis, quantitative research findings from a large number of studies were compiled. A meta-analysis is restricted to one single type of research finding and qualitative information cannot be used, but it is less subjective than narrative reviews and allows to investigate the effect of “moderator variables”, i.e. to identify variables which influence restoration success and hence, to identify effective measures and to describe favourable conditions for river restoration. The meta-analysis was complemented by a satellite topic on urban restoration to identify differences between the characteristics of urban and non-urban restoration projects.

Based on the results of the meta-analysis, the satellite topic, and other comprehensive reviews on river restoration already published in literature, the following conclusions were drawn. It is important to note that - as for all statistical analysis - it is not possible to infer causal relationships and hence, results should be interpreted with caution. Furthermore, for most results of this study, restoration success refers to an increase in the number of individuals and taxa simply because these metrics were reported in literature, but other metrics might be better suited to quantify success (e.g. stream-type specific conditions, functional approaches). It is strongly recommended to read the results and discussion sections before applying the results to avoid oversimplified interpretations.

In summary, it was possible to draw some first conclusion for river management from the evaluation of hydromorphological restoration based on existing monitoring data. However, monitoring data are still scarce and more robust, practical relevant, and quantitative results (e.g. thresholds) could be derived and river management would benefit from (i) original monitoring data, which would allow to use functional metrics to investigate the underlying processes and to infer causal relationships, (ii) full before-after-control-impact monitoring designs, which most probably would substantially decrease scatter in the datasets and analyses, (iii) a larger number of monitored projects, which easily could be accomplished since a large number of hydromorphological restoration measures will be implemented in the upcoming years, (iv) the availability of long-time monitoring data sets to investigate the effect of project age,which was identified as the most important variable affecting restoration success. A more intensive exchange and collaboration between river science and river management in planning monitoring programs is strongly recommended. This would offer a great opportunity to make fundamental advances in our understanding of how river restoration affects river hydromorphology and biota and to identify (cost)-effective restoration measures.

Overall, the effect of hydromorphological restoration on biota is positive but variability is high: Restoration in general has a positive effect on floodplain vegetation, ground beetles, macrophytes, fish, and invertebrates. Since variability is high, adaptive management approaches are recommended.

Restoration effect differs between organism groups:It cannot be expected that all organism groups benefit from restoration to the same extent. Results indicate that, in general, restoration effect on diversity is highest for terrestrial and semi-aquatic groups like floodplain vegetation and ground beetles, intermediate for macrophytes, lower for fish, and lowest for macroinvertebrates.

Restoration has a higher effect on the number of individuals than on the number of taxa:The effect of restoration is more pronounced on the number of fish and invertebrate individuals than on the number of taxa.

Restoration effect does only slightly differ between measures, i.e. there is no single “best” measure:There are no large differences in the overall effect of different measures but there is a tendency that terrestrial and semi-aquatic organism groups like floodplain vegetation and ground beetles as well as macrophytes benefit most from channel-planform measures and aquatic groups like fish and invertebrates from instream measures.

Urban restoration projects do not substantially differ in respect to the pressures occurring and the measures applied: Urban restoration projects were mainly applied in small rivers and length of the restored reaches was shorter compared to non-urban restoration projects. However, approaches towards (sub)urban and non-urban river rehabilitation practices were similar.

Urban restoration projects are rated less successful compared to non-urban projects by scientists and river managers: …which is probably due to the low absolute effect, which is rated as a failure, but which often is a high relative effect since urban river start from a low base, and hence, should not be assessed too negative.

Conditions which favour restoration success can be identified but restoration outcome cannot be predicted: Restoration success is especially high for the different organism groups under specific conditions (see section 2.2.5) but the variance explained by the models is too low and low sample size restricted the use of rigorous statistical tests to really predict the restoration outcome.

Overall, restoration success most strongly depends on project age, river width, and is affected by agricultural land use:Success is generally lower but restoration still has a positive effect in catchments dominated by agricultural land use. Since land use is a proxy for e.g. water quality, there is an urgent need to identify the underlying causal relationships.Project age is the most important predictor affecting restoration success, but the direction of the effect of project age on restoration success differs between organism groups (no simple increase of effect with time). There is an urgent need for long-time monitoring to investigate the restoration effect over time, to better understand the trajectories of change induced by restoration measures, and to identify sustainable measures which enhance biota in the long-term.