One of the main approaches in REFORM is to review existing restoration data to learn from the past and improve river management in the future. 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, the monitoring results of several projects are available. Some narrative reviews have already been published, but a comprehensive quantitative, so-called “meta-analysis”, which summarises the findings of these existing studies was lacking. This meta-analysis took place in REFORM (deliverable 4.2) and aimed at quantifying restoration success; identifying catchment, river reach, and project characteristics which influence (either constrain or enhance) the effect of restoration; and deriving recommendations for river management.

Figure 1: Two typical restoration projects included in the database, focusing on channel planform (left, © A. Lorenz) and instream channel features (right, © W. Klein).

A unique dataset has been compiled, containing 353 entries on the effect of restoration derived from 120 projects. The projects covered a wide range of different river types, river sizes, catchment land use and project age, but most restored reaches were rather short (< 2.6 km) and implemented in the second-last decade (10-90^th percentile range 1991-2005). In most projects, multiple measures have been applied to restore instream habitats, channel-planform, the riparian or floodplain area and/or river continuity, whereas sediment, hydrological and flow restoration measures were virtually missing in the dataset (Figure 1).

Although the meta-analysis is the most comprehensive study to date, it was still restricted in two ways. First, the study focused on the effect of restoration on the diversity and abundance of fish, macroinvertebrates and macrophytes, simply because only these biological metrics and organism groups were investigated in a larger number of studies, allowing for a statistical analysis. Second, restoration projects were not intended and designed to be scientific experiments. Therefore, they often applied multiple measures and relatively simple and short-term Control-Impact monitoring schemes (comparison of restored and unrestored reaches), making it difficult to disentangle the effect of single restoration measures and the effect of restoration from general trends in the catchment.
Nevertheless, based on the results and other existing reviews, the following conclusions can be drawn:

• 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.
• Restoration effect differs between organism groups: In general, restoration effect on diversity in the medium-term is highest for terrestrial and semi-aquatic groups like floodplain vegetation and ground beetles, intermediate for macrophytes, lower for fish and lowest for macroinvertebrates. This is especially true for channel-planform measures like re-meandering and widening/re-braiding.
• Restoration has a higher effect on the number of individuals than on the number of taxa: This indicates that, in general, it is easier to increase the number of individuals in the restored reach than establishing new taxa.
• Restoration effect only slightly differs 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, ground beetles and macrophytes benefit most from channel-planform measures and aquatic groups like fish and invertebrates from instream measures.
• Conditions which favour restoration success can be identified but ecological outcome cannot be predicted with great certainty.
• Overall, restoration success most strongly depends on project age, river width, and is affected by agricultural land use: Project age is the most important predictor affecting restoration success (Figure 2), but the direction of the effect of project age on restoration success differs between organism groups (no simple increase of restoration effect with time).

Figure 2: Relative importance of catchment, river reach and project characteristics for restoration success.

In summary, it was possible to draw some initial and important conclusions for river management from the evaluation of hydromorphological restoration based on existing monitoring data. However, monitoring data are still scarce. More robust, practical, relevant and quantitative results (e.g. thresholds) could be derived and river management would benefit from:

• Original monitoring data, which would allow the use of functional metrics to investigate the underlying processes and to infer causal relationships.
• Full before-after-control-impact (BACI) monitoring designs, which most probably would substantially decrease scatter in the datasets and analyses.
• 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.
• 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.

The full report “Evaluation of hydromorphological restoration from existing data” is available here.

Author: Jochem Kail (IGB/UDE)

About "PhD research in REFORM"

In the newsletter items dedicated to PhD research in REFORM, PhD students introduce the topic and the initial results of their research.

Introduction

Rivers in Spain are highly altered due to the existence of more than 1200 large dams. There are few detailed studies on the cumulative effect that this intense flow regulation has over time and space. Today, it is known that the presence of dams not only modifies flow regime but greatly modifies sediment flows causing different responses in channel morphology, habitat quality and dynamics of riparian vegetation. Particularly in Spain, the diversity of bio-geographic gradients and river types, together with the different geological contexts and substrates result in a multiplicity of responses to modifications. Therefore, specific case studies are needed to consider the interaction between river types and pressures (e.g. land use of the watershed).

Objectives

The general objective of this thesis is to improve the knowledge on bio-geomorphic responses of rivers to human pressures and to contribute to their management based on the Water Framework Directive. Particularly, this work analyses the homogenisation of channel morphology and significant changes in the riparian corridor that have been observed in the Upper Esla Basin in the last 50 years. This fluvial system is affected by intensive flow regulation for irrigation purposes that, in Mediterranean regions, entails an increase in low summer flows (in comparison to natural conditions). Current knowledge indicates that flow regulation initiates riparian vegetation changes through flood reduction that could decrease successful recruitment of native riparian pioneer species. Our objectives were to analyse changes in riparian vegetation patterns in time and to verify if these changes could be associated with flow regulation.

Figure 1: Curueño River, Summer 2013 (photo: Marta González del Tánago)

Approach

River reaches where natural vegetation remains were selected for the research sites. The initial analysis was conducted on regulated reaches of the Esla and the Porma rivers and one un-regulated reach of the Curueño River (Figure 1). Mean daily discharge data were analysed considering pre- and post- dam periods. A set of aerial photographs (1956 - 2011) were analysed in each studied reach and changes in active channel and vegetation were quantified. Fieldwork was done to analyse current landforms and woody vegetation structure, the results being the identification of seven different landform types and the collection of data on presence and size of woody vegetation along 3 transects in each reach.

Preliminary Results

Active channel area has decreased steadily over the studied period. In 1956, the active channel area was largest in all cases. In the successive pictures, channel narrowing and loss of the initial active channel area together with vegetation encroachment and poplar plantations is observed in all rivers (Figure 2).

Figure 2:  Example of a part of the Porma river reach that exhibited a pre-dam multithread channel with wide banks in 1956 (left) and a single channel in 2011 (center) with human pressures (right) (figure: Vanesa Martínez-Fernández).

The most active landforms, bare gravels with frequent recruitment of pioneer species, were absent in the regulated reaches (Esla, Porma). Multivariate analysis revealed that seedlings of pioneer (Salicacea) species commonly related with active bars were only found in the Curueño River. However, larger individuals of these pioneer species are found in old channel banks of the Porma and Esla rivers, where flood disturbance has disappeared after flow regulation. Since the operation of the dams, the maximum annual discharge has decreased (about 70%) and minimum annual discharge has risen. In both regulated rivers, no flood events exceeding the 2-years return period (incidence based on natural flow regime characteristics) have occurred since the operation of the dams. In the Curueño River, flood disturbance has continued to occur but with smaller magnitude. On the other hand, an increase of the minimum flows during summer months has occurred in the regulated rivers due to irrigation purposes. This contrasts with the natural Mediterranean seasonality presented by the Curueño River, where low flows still occur in the summer months.

Our preliminary results show narrowing processes in the studied rivers associated to a decrease of flood disturbance and highlight the risk that native riparian forest will not rejuvenate but will age and disappear in due course under regulated flow regimes.

Future work will include more reaches and other type of catchments and landscape variables linking the evolution and trends of the fluvial system to human pressures that have modified natural conditions.

Author:  Vanesa Martínez-Fernández, UPM (Polytechnic University of Madrid)

On 2 June 2014, a National Stakeholder Workshop on "Restoring Rivers: Experiences and methodological advances” took place in the city of Seville, at the Pabellón de Méjico (University of Seville) organised by the UPM (Polytechnic University of Madrid). The general aim of the workshop was to raise awareness and interest of water managers and stakeholders on the REFORM Project and its results. The workshop was opened by Dr. Leandro del Moral, Professor at the University of Seville, Dr. Manuel Romero, President of the Guadalquivir Water District and Marta González del Tánago from UPM. The opening speakers highlighted the need for efficient water use and the advances made in recent years in the immediate vicinity of Seville.

Figure 1: Workshop plenary room (photo: Tom Buijse)

The workshop focused on the methodological framework developed at European level within the REFORM project to ensure the success of restoration measures in rivers. This framework includes the advances in knowledge on hydro-morphological and ecological processes and its relation to sustainability and ecosystem services.  The proposed advances are expected to be useful for the new River Basin Management Plans (RBMPs) in the context of the Water Framework Directive.

Figure 2: Interaction among participants (photo: Tom Buijse)

The workshop programme started with an introduction to REFORM followed by “setting the Mediterranean scene” presentations by Spanish experts on river restoration practices in the Spanish Mediterranean. Next came several presentations by internationally renowned researchers working together within the REFORM Project. The presentations were followed by an open discussion of the presented topics, with participant support through direct English-Spanish translation to tackle the language barrier. Attendance was free of charge and approximately 50 participants attended the workshop. Brunch and coffee sessions were offered during the duration of the workshop, which helped maximise discussion and interactions between participants. Participating stakeholders were mainly water managers from the water administration of Andalucía as well as university researchers, environmentalists, consultants and farmers.

For further information, please refer to the programme of the workshop (see below) or follow this link to access the speakers' presentations.

Author: Marta González del Tánago, UPM (Polytechnic University of Madrid)

The 380 km long River Spree drains about 10.000 km² into the rivers Havel and later Elbe. Since the early middle age, the lower Spree was heavily modified to facilitate flood protection, water mills, agricultural land use and inland navigation. The discharge (avg 36 m³/s) was artificially increased by ground water which was pumped into the River Spree during open-cast lignite mining activities in the middle and upper reaches to keep the mining area dry. In response, cross-sections were widened and deepened, banks stabilized and meanders cut-off, later on reservoirs and by-pass canals were constructed. As a consequence, the river lost its natural variability of the flow regime. The absence of flow spates allured intensive agriculture and building of houses and infrastructure in the former floodplain.

In the 1990s, the lower River Spree was characterised by eutrophic conditions (0.7-3.4 mg/L total nitrogen and 70-180 g/L total phosphorous), uniform channel form, almost lacking flow dynamics and the corresponding biota, phytoplankton, limnophilic and eurytopic invertebrates and fish. Therefore, the water authorities in cooperation with numerous scientists developed a comprehensive management concept to improve the quality of a 35 km long river section, the so-called Müggelspree. The concept comprised reductions in nutrient loading, re-meandering, raising the riverbed, removal of bank protections, and allowing for flow spates.

Figure 1: Re-meandering – the former backwater has been reconnected at both sides and the former main channel blocked by a dam (near the left, upstream opening) (photo: Isabell Hiekel).

However, this concept was opposed by a single farmer and very few inhabitants afraid of flood damages. Reluctance of private land owners and political concerns impeded the essential acquisition of adjacent floodplain areas by public authorities. As a result, flood pulses, bank erosion, bedload transport and any other natural, self-sustaining hydromorphological processes apart from the ongoing river bed incision are still lacking today.

Finally, four of about 30 backwaters were reconnected between 2004 and 2008. They became the new main stem whereas the former artificial cut-offs were blocked (Figure 1). In this way, the river length increased by about 2.7 km. However, these renewed meanders were completely dredged according to the main channel cross section, artificially wide and rather homogeneous, to ensure the same discharge capacity for flood protection. The former main channel was completely blocked near the opening to force the whole discharge through the new meander and the section up to the downstream end of the new meander remained a static water body.

The first two backwaters, reconnected in 2004 and 2005, have been extensively monitored for restoration success between 2005 and 2007. Despite all limitations mentioned, some positive effects of re-meandering were found. The flushed meander provided a much more diverse bed substrate than the former backwater. Habitat heterogeneity had adjusted to the main channel conditions already in August 2006 (Lorenz & Leszinski subm.). An experimental moderate flow spate slightly eroded the outer bank. In the new meanders the flow tolerant macrophyte species dominating the main channel started colonisation with slight delay on the newly dredged bottom substrates (Grünert et al. 2007). The formerly extinct rheophilic thick-shelled river mussel (Unio crassus) had settled and the number of sensitive EPT taxa (benthic invertebrates of the families Ephemeroptera, Plecoptera, Trichoptera) as well as the share of typical riverine species increased (Lorenz & Leszinski subm.). The community structure of fish, macroinvertebrates and aquatic vegetation in the new meanders adjusted to those in the main stem within three years (Grünert et al. 2007, Wolter 2010, Lorenz & Leszinski subm.). However, problems became especially obvious for fish (Figure 2): the simple adjustment to the existing main channel communities will not ecologically improve the river. Sensitive indicator fish species for river health are still declining in both the old main channel and the re-meandered stretches (Wolter 2010). Accordingly, the fish-based assessment score did not improve, because the essential bottlenecks limiting riverine fish species were not addressed by re-meandering as implemented here (Wolter 2010).

Figure 2: Observed fish densities in the main channel compared to the rehabilitated new meanders of A) total fish, B) rheophilic fish, and C) lithophilic fish (data from Wolter 2010).

All in all, re-meandering of rivers is a common and widely applied river rehabilitation measure. However, if not properly implemented, its effect may stop at the adjustment of a former backwater species community to those of the main channel without further improvement. Ecologically effective re-meandering cannot be static in morphology and statistically neutral in flood risk. In fact, it will always increase the theoretical flood risk by locally narrowing river cross sections. Ecologically successful re-meandering requires allowing for side erosion, depth and width variations as well as channel narrowing to achieve diverse flow patterns, sediment transport and sediment sorting, thereby creating the essential microhabitat patches for typical, sensitive riverine species.  

In the frame of the REFORM project, two of the meanders of the Müggelspree are compared with straight sections of the lower Spree regarding hydromorphology, species structure of main groups of organisms, food web interactions and connectivity of river and floodplain. The data has not undergone a final analysis yet, but ultimately the River Spree case study might serve as example for inconsequent and thus inefficient restoration efforts.

Figure 3: Map of the River Spree catchment up to its Mouth into the River Havel. The insert shows the stretch “Müggelspree” where the red ellipses indicate the re-meandered sites.

References

Grünert U, Hilt S, Pusch M, Gelbrecht J. 2007. Entwicklungspotential der Makrophytenvegetation in der Unteren Spree nach Renaturierungsmaßnahmen. Naturschutz und Landschaftspflege in Brandenburg 16, 41-47.

Lorenz S, Leszinski M. subm. Re-meandering technique determines colonization success of macroinvertebrates in a German lowland river. Hydrobiologia.

Wolter C. 2010. Functional vs scenic restoration – challenges to improve fish and fisheries in urban waters. Fisheries Management and Ecology 17, 176-185.

Authors: Jan Köhler & Christian Wolter (IGB)