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Thur

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Key features of the case study

The recent rehabilitation of the river Thur has been one of the prominent restoration programmes in Switzerland. It is a clear example of restoration management in Switzerland, providing information concerning restoration effects, and stimulating current and future studies. Today, in Switzerland a strong political willingness exists to increase the space available for rivers, with the hope to improve their protecting role against floods and their ecological state at the same time.

Site description

The Thur is a 127 km long river flowing from the Swiss Alps in the north east of Switzerland. It is a tributary of the Rhine river which also originates in the Swiss Alps and ends in the North Sea. The Thur is the largest Swiss river without natural or artificial reservoirs along its course. Its discharge is similar to unregulated alpine rivers, the water level can therefore increase rapidly during rain events or snowmelt. For agricultural purposes and protection of residential areas, the Thur river was embanked (late 19th century) and its natural floodplain was drastically reduced.

Before river engineering in 1890, the river braided, gravel bars were frequent, islands were present, natural alluvial forest accompanied the river. The complexity and heterogeneity of the habitats were maintained by the natural dynamics of the river itself. Its regulation created a monotonous channel surrounded by high levees and accompanied by embankments which fixed the river. New agricultural land was gained behind the levees for crop production, and between the levees and the embankments for grazing. At the same time, villages and cities were better protected from flood events during which discharge can raise to 1130 m³/sec (in 1999), far from the mean annual flow of 47 m³/sec. The stabilisation of the river bed also permitted to extract drinking water filtrated in the sediment. These human interventions had direct consequences on the hydromorphological conditions, the ecosystem functioning, and its biodiversity. Gravel bars disappeared, secondary channels terrestrialised, characteristic riverine species became rare, floodplain related species disappeared and the complexity of the system was largely reduced (see photos below).

Degraded section with bank fixation and levees (orange arrows) and narrow floodplains (P. Reichter, A. Paillex)

Measures selection

To date, important efforts are made to restore a natural morphology within the river. The aims are to increase natural protection against floods, and to increase natural processes and habitat diversity. Parts of the Thur river were restored, as for example in 2002 on a 1.5 km stretch close to the villages of Niederneunforn and Altikon (i.e. stretch selected as the restored site for the Reform project, Figure 2). The river was widened on one or both sides of the main river channel. Along the course of the river, embankments were removed to provide a larger space to the river. Additional wood structure were added to enhance the ability of the river to braid. The dynamic processes were expected to be return, with natural patterns of erosion and deposition, better connection between the main river channel and the floodplain, and recreation of secondary channels. Overall, an increase of instream and terrestrial habitats diversity was expected, leading to an increase in biotic richness and diversity, both in the river and on the banks.

Restored gravel bar (left), stagnant water body (right), and overview of the restored section (bottom)(A. Paillex, H. Mottaz)

Hydromorphological and ecological response

To date, hydromorphological and biological indicators both suggest an increase in river quality and conditions after restoration. As a result from river restoration, we can observe that gravel bars were recreated, secondary channels appeared, and zones of erosion and deposition now co-exist at the scale of the restored reach. Less visible, but measurable with appropriate methods, are the biological improvements in the restored reach compared to regulated stretches. The richness in benthic invertebrates, fish, ground beetles, aquatic vegetation and floodplain vegetation has increased in the restored section (see table below). According to a valuation of richness and of threatened and invasive species, the improvement was important for fish and ground beetles, intermediate for floodplain vegetation, and less significant for benthic invertebrates and aquatic plants. A positive effect is that threatened species (e.g. the fish Chondrostomas nasus) took advantage of this restoration project, while a negative side effect is the occurrence of an invasive aquatic plant in the restored site (i.e. Elodea nuttallii). A combined valuation of biological and morphological conditions indicates an overall positive effect of restoration on the ecological state of the river.

Socio-economic aspects

A positive effect of this enhanced naturalness is an increased number of people visiting the river, especially on sunny days. This is supported by an easier access to the river and more opportunities for recreational activities.

Outlook

However, there is still a lot to do to sustain this improvement for the long term. Today, we may have to prevent invasive species from colonising the recovered habitats, and improve the water quality to increase the biological success. Finally, it is important to recall that this restored part is only 1.5 km long while the majority of the remaining 130 km of the river are still totally embanked.

Contact person within the organization

Prof. Mario Schirmer or Dr. Amael Paillex E-mail: Mario.Schirmer or Amael.Paillex[at]eawag.ch Organization Name: Eawag, Swiss Federal Institute of Aquatic Science and Technology Überlandstrasse 133 CH 8600 Dübendorf Switzerland

Extra background information

Link to project website:

http://www.cces.ethz.ch/projects/nature/Record http://www.rivermanagement.ch/en/welcome.php http://www.rhone-thur.eawag.ch/

References

aEawag, WSL, ETHZ, EPFL, 2005: Integrales Gewässermanagement - Erkenntnisse aus dem Rhone-Thur Projekt http://www.rivermanagement.ch

Schirmer, M., Luster, J., Linde, N., Perona, P., Mitchell, E. A. D., Barry, D. A., Hollender, J., Cirpka, O. A., Schneider, P., Vogt, T., Radny, D., and Durisch-Kaiser, E. 2014 Morphological, hydrological, biogeochemical and ecological changes and challenges in river restoration – the Thur River case study, Hydrol. Earth Syst. Sci., 18, 2449-2462, http://www.hydrol-earth-syst-sci.net/18/2449/2014/hess-18-2449-2014.html

European Centre for River Restoration (ECRR). 2008. ECRR Addressing practitioners.

Related Measures

• Reduce anthropogenic flow peaks
• Shorten the length of impounded reaches
• Link flood reduction with ecological restoration
• Establish environmental flows / naturalise flow regimes
• Widen water courses
• Allow/increase lateral channel migration or river mobility
• Remeander water courses
• Shallow water courses
• Initiate natural channel dynamics to promote natural regeneration
• Remove sediments
• Remove bank fixation
• Remove or modify in-channel hydraulic structures
• Recreate gravel bar and riffles
• Develop riparian forest
• Adjust land use to develop riparian vegetation
• Revegetate riparian zones
• Remove bank fixation
• Lower river banks or floodplains to enlarge inundation and flooding
• Remove hard engineering structures that impede lateral connectivity
• Set back embankments, levees or dikes
• Retain floodwater

Related Pressures

• Groundwater abstractions
• Artificial barriers downstream from the site
• Artificial barriers upstream from the site
• Alteration of instream habitat
• Embankments, levees or dikes
• Loss of vertical connectivity
• Alteration of riparian vegetation
• Channelisation / cross section alteration