Key messages

A literature review of past practices formulated in 10 key findings

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. WISER conceptual models) 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
2. REFORM deliverable D4.3
3. Downs & Kondolf (2002)
4. Williams & Brown (2014)

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
2. REFORM deliverable D2.3
3. Acuna et al. (2013)

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
2. REFORM deliverable D4.2.
3. Haase et al. (2013)
4. Jähnig et al. (2009)
5. Januschke et al. (2014)
6. Januschke et al. (2011)

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.
2. Kail et al. (2012)
3. Stoll et al. (2014)
4. Sundermann et al. (2013)
5. Tonkin et al. (2014)

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

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
2. REFORM deliverable D4.2
3. Haase et al. (2013)
4. Jähnig et al. (2009)
5. Januschke et al. (2009) (in German)
6. Lorenz et al. (2012)
7. Miller et al. (2010)
8. Roni et al. (2015)

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
2. Palmer et al. (2010)

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
2. REFORM deliverable D4.2
3. Schmutz et al. (2014)
4. Lorenz et al. (2012)
5. Whiteway et al. (2010)
6. Haase et al. (2013)

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
2. REFORM deliverable D4.2

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
2. Radinger & Wolter (2015)
3. Stoll et al. (2013)
4. Stoll et al. (2014)
5. Sundermann et al. (2011)
6. Tonkin et al. (2014)
7. Winking et al. (2014)