Key messages

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A literature review of past practices formulated in 10 key messages

Project formulation includes the final selection of specific rehabilitation measures, which is critical and still difficult due to the limited knowledge on the effect of (hydromorphological) river restoration and the fact that effects depend on catchment, river, and project characteristics. In REFORM, the current knowledge was summarized in ten key messages or principles based on a comprehensive literature review and complementary field studies. This is only a starting point and river managers should gather as much information as possible, consulting the underlying primary literature and other reviews (e.g. http://www.wiser.eu/results/conceptual-models/index.php) as well as visiting similar restoration projects.

General: Monitor and adjust your project – nobody can fully predict restoration outcomes

Key finding - River restoration has an overall positive effect but variability is high.

  • Overall, hydromorphological river restoration has a positive effect on different aquatic organism groups (e.g. macrophytes, fish, macroinvertebrates). However, variability is very high and a large number of projects had no or even a negative effect on species richness, diversity, or abundance.
  • A substantial share of the variation can be explained statistically but we are still far from really predicting the effect of restoration based on the project, river, and catchment characteristics.


Conclusions - It is necessary to apply adaptive management approaches.

  • The high variability of restoration effects within and between organism groups and our inability to really predict restoration outcomes stresses the need for post project appraisals and adaptive management approaches as described in Downs & Kondolf (2002) and Williams & Brown (2014).
  • The key aspect of adaptive management or the PDCA cycle is to include a monitoring and planning to adjust the restoration project if necessary from the beginning.


Further reading

  1. REFORM deliverable D4.2 “Evaluation of hydromorphological restoration from existing data”
  2. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  3. Downs, P.W., Kondolf, G.M (2002). Post-project appraisals in adaptive management of river channel restoration. Environmental Management, 29, 477-496.
  4. Williams, B.K., Brown, E.D. (2014). Adaptive management: From more talk to real action. Environmental Assessment, 53, 465-479.


Societal benefits: Restoration pays – it increases ecosystem services.

Key finding - Restored reaches provide higher ecosystem services compared to unrestored reaches.

  • Total ecosystem service value for restored reaches is usually higher compared to unrestored reaches due to higher cultural and regulating services, especially in densely populated regions.
  • The increase in value due to restoration is often several times higher than land rent and surpasses the costs of restoration over realistic timeframes.


Conclusions - Society benefits from restoration and changes in ecosystem services should be considered in the assessment of river restoration projects.

  • River restoration enhances societal benefits, especially cultural and regulating services.
  • Therefore, the effect of river restoration projects should be monitored and assessed in a holistic way, including biota (semi-terrestrial and terrestrial organism groups see key message “organism groups”) as well as ecosystem services. Rivers are socio-ecological systems.


Further reading

  1. REFORM deliverable D4.4 “Assessing the societal benefits of river restoration using the ecosystem services approach” and related publication in an upcoming special issue of Hydrobiology.
  2. REFORM deliverable D2.3 “Analytical framework ecosystem services: Valuing the ecosystem services provided by European river corridors – an analytical framework”.
  3. Acuna, V., Ramon Diez J., Flores, L., Meleason, M., Elosegi, A. (2013). Does it make sense to restore rivers for their ecosystem services? Journal of Applied Ecology, 50, 988-997.


Organism group: Terrestrial and semi-aquatic species benefit most from restoration.

Key finding - Effect on floodplain vegetation, ground beetles, macrophytes > fish > macroinvertebrates.

  • Not all organism groups benefit from restoration to the same extent. In general, restoration effect is highest for terrestrial and semi-aquatic groups like floodplain vegetation and ground beetles, intermediate for macrophytes, lower for fish, and lowest for macroinvertebrates.


Conclusions - Consider aquatic AND terrestrial organism groups.

  • The effect of river restoration projects on biota should be monitored and assessed in a holistic way, including semi-terrestrial and terrestrial organism groups. Terrestrial (floodplain) and aquatic ecosystems are closely linked and cannot be considered separately.


Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  2. REFORM deliverable D4.2 “Evaluation of hydromorphological restoration from existing data”.
  3. Haase, P., Hering, D., Jähnig, S. C., Lorenz, A. W., Sundermann, A. (2013). The impact of hydromorphological restoration on river ecological status: a comparison of fish, benthic invertebrates, and macrophytes. Hydrobiologia, 704, 475-488.
  4. Jähnig, S. C., Brunzel, S., Gacek, S., Lorenz, A. W., Hering, D. (2009). Effects of re-braiding measures on hydromorphology, floodplain vegetation, ground beetles and benthic invertebrates in mountain rivers. Journal of Applied Ecology, 46, 406–416.
  5. Januschke, K., Jähnig, S. C., Lorenz, A. W., Hering, D. (2014). Mountain river restoration measures and their success(ion): Effects on river morphology, local species pool, and functional composition of three organism groups. Ecological Indicators, 38, 243–255.
  6. Januschke, K., S. Brunzel, P. Haase, D. Hering (2011). Effects of stream restorations on riparian mesohabitats, vegetation and carabid beetles: a synopsis of 24 cases from Germany. Biodiversity and Conservation, 20, 3147–3164.


Biological metric: Restoration results in a higher number of individuals but few new species.

Key finding - Effect on species abundance > effect on the number of species.

  • In many projects, restoration had a high effect on species abundance, i.e. increased the number of individuals. In contrast, it often did not substantially increase the number of taxa, i.e. richness (REFORM deliverable D4.2). [Link to deliverable on REFORM website].
  • The higher abundance might be due to a real increase in reproduction and/or the restored reach attracts individuals from adjacent reaches.


  • Among others, the small increase in the number of species might have three main reasons: First, there are no nearby source populations or migration barriers hinder colonization of the new habitats. Recent empirical and modelling studies have shown that the species pool and source populations for (re-) colonization are often sparse and limiting the effect of restoration on biodiversity (Stoll et al. 2014, Tonkin et al. 2014). Second, restoration failed to create the right habitats (see key factor “habitats”). Third, other, large-scale pressures are still limiting colonization (e.g. water quality, fine sediment input, Sundermann et al. 2013; Kail et al. 2012).


Conclusions - It is easier to increase the abundance of existing than to establish new species.

  • An increase in species abundance can be expected and considered a success. Don’t expect that many new and rare species will establish in your restored reach unless you restored the right habitats, addressed all other, large-scale pressures, and source populations are located nearby for rapid colonization of the new habitats.


Further reading

  1. REFORM deliverable D4.2 “Evaluation of hydromorphological restoration from existing data”.
  2. Kail, J., Arle, J., Jähnig, S.C. (2012). Limiting factors and thresholds for macroinvertebrate assemblages in European rivers: Empirical evidence from three datasets on water quality, catchment urbanization, and river restoration. Ecological Indicators, 18, 63–72.
  3. Stoll, S., Kail, J., Lorenz, A.W., Sundermann, A., Haase, P. (2014). The Importance of the Regional Species Pool, Ecological Species Traits and Local Habitat Conditions for the Colonization of Restored River Reaches by Fish. PlosOne, 9, e84741.
  4. Sundermann, A., Gerhardt, M., Kappes, H., Haase, P. (2013). Stressor prioritisation in riverine ecosystems: Which environmental factors shape benthic invertebrate assemblage metrics? Ecological Indicators, 27, 83–96.
  5. Tonkin, J.D., Stoll, S., Sundermann, A., Haase, P. (2014). Dispersal distance and the pool of taxa, but not barriers, determine the colonisation of restored river reaches by benthic invertebrates. Freshwater Biology, 59, 1843–1855.


Biological metrics: Restoration rather affects specific species or traits than increasing the mere total number of species.

Key finding - Restoration has a larger effect on specific species or traits than on the mere total number of species.

  • In general, the effect of restoration on community structure, specific species, and traits (e.g. flow and substrate preferences) was more pronounced compared to its effect on the mere total number of species (richness).
  • These changes in community structure potentially indicate specific functional changes caused by river restoration and should be used in future to increase our understanding how restoration measures affect aquatic ecosystems.


Conclusions - Functional aspects should be used to assess restoration projects and investigate restoration effects.

  • Future monitoring and studies should focus more on functional aspects (e.g. species traits, community structure) to investigate how river restoration affects river hydromorphology and biota, which would offer a great opportunity to make fundamental advances in restoration ecology and to identify (cost)-effective restoration measures.
  • Restoration projects should also aim at restoring ecosystem functions and focus more on traits besides assessing restoration success based on the effect on species richness and diversity.


Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.


Measures: There is no single “best measure” but widening generally has a high effect.

Key finding - Similar effects of different measures on richness/abundance; high effect of widening especially on terrestrial and semi-aquatic species.

  • There are no large differences in the overall effect of different measures on the richness and abundance of different organism groups.
  • However, 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, especially from river widening/rebraiding and aquatic groups like fish and invertebrates from instream measures like large wood addition.


Conclusions - Widening is one of the most effective measures for floodplain vegetation, ground beetles, and macrophytes, instream measures also have significant effects especially on fish and macroinvertebrates.

  • Widening (removing bed and bank fixation, flattening river banks, and in some projects considerably widening the cross-section) is one of the most effective restoration measure, especially for terrestrial vegetation, ground beetles, and macrophytes.
  • Instream measures in the wetted channel like large wood addition also have significant positive effects, especially for fish and invertebrates.


Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  2. REFORM deliverable D4.2 “Evaluation of hydromorphological restoration from existing data”.
  3. Haase, P., Hering, D., Jähnig, S. C., Lorenz, A. W., Sundermann, A. (2013). The impact of hydromorphological restoration on river ecological status: a comparison of fish, benthic invertebrates, and macrophytes. Hydrobiologia, 704, 475-488.
  4. Jähnig, S. C., Brunzel, S., Gacek, S., Lorenz, A. W., Hering, D. (2009). Effects of re-braiding measures on hydromorphology, floodplain vegetation, ground beetles and benthic invertebrates in mountain rivers. Journal of Applied Ecology, 46, 406–416.
  5. Januschke, K., Sundermann, A., Antons, C, Haase, P., Lorenz, A.W., Hering, D. (2009). Untersuchung und Auswertung von ausgewählten Renaturierungsbeispielen repräsentativer Fließgewässertypen der Flusseinzugsgebiete Deutschlands. Verbesserung der biologischen Vielfalt in Fließgewässern und ihren Auen. Schriftenreihe des Deutschen Rates für Landespflege, 82, 23-39.
  6. Lorenz, A. W., Korte, T., Sundermann, A., Januschke, K., Haase, P. (2012). Macrophytes respond to reach-scale river restorations. Journal of Applied Ecology, 49, 202–212.
  7. Miller, S.W., Budy, P., Schmidt, J.C. (2010). Quantifying macroinvertebrate responses to in-stream habitat restoration: applications of meta-analysis to river restoration. Restoration Ecology, 18, 8–19.
  8. Roni, P., Beechie, T., Pess, G., Hanson, K. (2015). Wood placement in river restoration: fact, fiction, and future direction. Canadian Journal of Fisheries and Aquatic Science, 72, 466-478.


Habitats: It is important to restore specific habitats not necessarily mere habitat diversity.

Key finding - Ground beetles benefit from specific habitats, macroinvertebrates are related to microhabitat diversity.

  • Ground beetle richness does not depend on mere habitat diversity but the presence of specific habitats: sparsely vegetated sand and gravel bars.
  • Macroinvertebrate richness increases with microhabitat diversity but is then limited by other stressors.


Conclusions - It is crucial to restore specific habitats at spatial scales relevant for biota (e.g. microhabitats).

  • It is not necessarily most important to increase the mere number of habitat types (e.g. habitat diversity) but to restore specific habitats which are of special importance or presently limiting colonization.
  • It is crucial to ensure that restoration measures create habitats at spatial scales relevant for biota (e.g. substrate diversity at the microhabitat scale for invertebrates). The outcome of restoration projects might be visually appealing (visible changes in macro- and mesohabitat diversity) but fail to increase microhabitat diversity.


Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  2. Palmer, M.A., Menninger, H., Bernhardt, E.S. (2010). River restoration, habitat heterogeneity and biodiversity: a failure of theory or practice? Freshwater Biology, 55, 205–22.


Project size and age: Small restoration projects work but better act big and long-term.

Key finding - Projects investigated were too small to detect an effect of project size; contrasting results for project age.

  • In REFORM, we expected higher restoration effects in larger projects since longer reaches might mitigate the influence of large-scale pressures like fine sediment input and provide a minimum area for hydromorphological processes to act and viable populations to establish. Restoration had positive effects even in small restoration projects but effects did not increase in larger projects. Most probably, even the largest projects investigated in our study were still too small to increase restoration effects and projects have to be larger to show big effects.
  • Furthermore, we expected that restoration effects increased over time, and hence being larger in older projects. In the 20 REFORM case-studies, restoration effect did not depend on project age or showed non-linear trends. In contrast, project age was one of the main factors influencing restoration outcome in the literature review. However, the effect of restoration did not simply increase with time but showed different and non-linear relationships.


Conclusions - Future projects should be larger to benefit from project size; project age has to be investigated further.

  • Even small restoration projects can have a positive effect on some organism groups. Slightly larger projects don’t have larger effects. Most probably, restoration projects implemented in the past were simply too small to benefit from possible positive mitigating effects of project size and big projects are needed for big effects.
  • The effect of restoration does not simply increase over time but changes non-linearly and might even decrease. This stresses the need to further investigate the effect of restoration over time in future studies to better understand the trajectories of change induced by restoration measures, and to identify sustainable measures which enhance biota in the long-term.


Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  2. REFORM deliverable D4.2 “Evaluation of hydromorphological restoration from existing data”.
  3. Schmutz, S., Kremser, H., Melcher, A., Jungwirth, M., Muhar, S., Waidbacher, H., Zauner. G. (2014). Ecological effects of rehabilitation measures at the Austrian Danube: a meta-analysis of fish assemblages. Hydrobiologia, 729, 49-60.
  4. Lorenz, A. W., Korte, T., Sundermann, A., Januschke, K., Haase, P. (2012). Macrophytes respond to reach-scale river restorations. Journal of Applied Ecology, 49, 202–212.
  5. Whiteway, S.L., Biron, P.M., Zimmermann, A., Venter, O. & Grant, J.W. (2010). Do in-stream restoration structures enhance salmonid abundance? A meta-analysis. Canadian Journal of Fisheries and Aquatic Science, 67, 831–841.
  6. Haase, P., Hering, D., Jähnig, S. C., Lorenz, A. W., Sundermann, A. (2013). The impact of hydromorphological restoration on river ecological status: a comparison of fish, benthic invertebrates, and macrophytes. Hydrobiologia, 704, 475-488.


Catchment characteristics: Slightly higher effects in gravel-bed mountain rivers with low land-use pressure.

Key finding - Projects in gravel-bed Mountain Rivers with low land-use pressure have a slightly higher effect on biota.

  • Agricultural land use in the upstream catchment and in the restored reach affects and limits the outcome of restoration. However, restoration has an overall positive effect even in catchments dominated by agricultural land use and hence, this does not question restoration in agricultural catchments in general.
  • The effect of restoration in gravel-bed mountain rivers tends to be higher for several organism groups compared to sand-bed lowland rivers, where restoration might even have a negative effect on specific organism groups and biological metrics (e.g. lower macroinvertebrate richness due to natural species-poor assemblages in sand-bed rivers).


Conclusions - Carefully consider catchment and river characteristics which might constrain restoration.

  • The outcome of restoration strongly depends on catchment and river characteristics and they have to be carefully considered.
  • Based on present knowledge, it seems that:
  • Restoration projects in gravel-bed mountain rivers with low land-use pressures have slightly higher effects on some organism groups (e.g. ground beetles) compared to projects in sand-bed lowland rivers with higher land-use pressure.
  • Restoration projects in agricultural catchments in general still have a positive effect on biota but they are more susceptible to failure.
  • The effect of restoration depends on different factors including large-scale pressures (e.g. water quality, fine sediment input), historical disturbances, a limited species pool and migration barriers hindering re-colonization of the restored section (see key factor “source populations”) as well as project age (see key factor “project age”). The knowledge on the effect of these factors on restoration success is still limited due to methodological problems or limited data availability and clearly merits further investigation.


Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  2. REFORM deliverable D4.2 “Evaluation of hydromorphological restoration from existing data”.


Catchment characteristics: Is the effect of restoration on aquatic organism groups mainly limited by missing source populations?

Key finding - The effect of restoration on fish and macroinvertebrates is potentially limited by the lack of source populations to colonize the new restored habitats.

  • Recent modelling and empirical studies indicate that the effect of restoration on fish and macroinvertebrates might be limited by the lack of source populations. Species might be extinct in a catchment or source populations located too far from the restored reach or dispersal hindered by migration barriers for species to colonize the new restored habitats within the limited time between restoration and monitoring.


Conclusions - The location of source populations has to be considered and this topic merits further investigation.

  • It is presently difficult to draw general conclusions based on the first empirical studies. This topic clearly merits further investigation since a limited re-colonization potential would need a completely different restoration strategy compared to habitat improvements. As a first rule of thumb, source populations for fish and macroinvertebrates should be located less than 5 km and 1 km upstream from the restored reach, respectively.

Further reading

  1. REFORM deliverable D4.3 “Effects of large- and small-scale river restoration on hydromorphology and ecology” and related publications in an upcoming special issue of Hydrobiology.
  2. Radinger, J., Wolter, C., 2015. Disentangling the effects of habitat suitability, dispersal and fragmentation on the distribution of river fishes. Ecological Applications, in press.
  3. Stoll S., Sundermann A., Lorenz A.W., Kail J., Haase P. (2013). Small and impoverished fish species pools are a main challenge to the colonization of restored river reaches. Freshwater Biology, 58, 664–674.
  4. Stoll, S., Kail, J., Lorenz, A.W., Sundermann, A., Haase, P. (2014). The Importance of the Regional Species Pool, Ecological Species Traits and Local Habitat Conditions for the Colonization of Restored River Reaches by Fish. PlosOne, 9, e84741.
  5. Sundermann A., Stoll S., Haase P. (2011). River restoration success depends on the species pool of the immediate surroundings. Ecological Applications, 21, 1962–1971.
  6. Tonkin, J.D., Stoll, S., Sundermann, A., Haase, P. (2014). Dispersal distance and the pool of taxa, but not barriers, determine the colonisation of restored river reaches by benthic invertebrates. Freshwater Biology, 59, 1843–1855.
  7. Winking, C., Lorenz, A.W., Sures, B., Hering, D. (2014). Recolonisation patterns of benthic invertebrates: a field investigation of restored former sewage channels. Freshwater Biology, 59, 1932-1944.