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Artificial floods are jointly responsible for deficits


“Hydropeaking“ in rivers and streams is becoming a problem for water ecology, particularly for fish. Rapid change in the release of water from alpine hydroelectric power stations leads to artificial discharge variations on a daily and weekly basis. Of Switzerland’s larger rivers, one in four is influenced by such water surges. Together with the wide-spread river training and channelisation, such intermittent flow is one of the main causes for the biological deficits that can be observed, for example, in the valleys of the Rhone. The success of revitalisation measures necessary in channelled waters depends on many factors but it may be put into question if the reduction of such surges is not considered.

The Rhone-Thur project is being carried out by the research institutes Eawag and WSL as well as by the hydrology institutes at the ETH in Zurich and the EPFL in Lausanne. The results concerning hydropeaking are now available. The rapid increase and ebbing away of water in riverbeds as a consequence of the use of hydropower has a distinct influence on ecological entegrity in addition to the river morphology deficits.

Broad Effects

In those sections subject to hydropeaking, researchers examining the Rhone have noted considerably scarce colonisation and lower biodiversity among fish and other water organisms than would be expected in non-affected sections. Among other things, the increase in the influence of hydropeaking between Brig and the Rhone Delta at Lake Geneva is reflected by the progressive decline in the occurrence of stone flies and ephemerides (photo) typically found in flowing waters. Typical effects of hydropeaking also include increased turbidity during the winter and short-term fluctuations in temperature. Water organisms are drifted away by rapidly increasing water levels during hydropeaking. When the water level drops again, they may ground at locations that quickly dry out.

This process may even increase when alpine rivers such as the Rhone are morphologically improved (revitalised). While shallow riparian areas of near-natural rivers are often the most multifariously colonised and well-used habitat of the whole river, the impoverished zones found in stretches subject to hydropeaking contain only those few plants and animals that can tolerate regular dry periods. Certain species of filamentous green algae and the larvae of specific caddy flies are such resistant organisms. The Rhone-Thur project also documented that even 30 kilometres downstream in the Rhone, hydropeaking is hardly attenuated (see diagram) and that its influence is not only limited to the river alone: Its effects are also to be noted in the underground and in the vicinity of the river, caused, for example, by changed infiltration rates into groundwater.

To be taken into account during revitalisation projects

Hydropeaking alone can not be made responsible for many of the ecological deficits found. This could be shown especially for the fish in the river Rhone: The today’s population of brown trout (the most frequently found fish by far) reflect the impoverished and monotonous morphology of the river and other anthropogenic influences (such as artificial stocking) just as much as hydropeaking. Therefore, the sole improvement of discharge i.e. without improvement of river morphology, would hardly suffice as a means of reinstating the ecological integrity of the river to the extent desired. On the other hand, however, hydropeaking complicates the matter and poses an additional, limiting condition on the revi-talisation of rivers and streams. This means, for instance, that when widening riverbeds, it must be assured that sufficient structural elements - gravel banks, dead wood etc. - are available and not only those areas are extended that always dry out after peak flow.

Hydropeaking attenuation is possible

Up to now, no legal regulations on hydropeaking exist either in Switzerland or in other countries. Regulations are imposed from case to case, for example on the renewal of operating licences. Both operational as well as structural measures can be taken into consideration to provide attenuation of peak flows. Results from the Rhone-Thur-project show that, when taking the economic viability of hy-dropower into account, the building of retention pools or underground storage systems is often the most sensible solution. Such pools already partly exist, for example at the Linthal (Glarus) and Amsteg (Uri) power stations in Switzerland and in Alberschwende in Austria. Depending on retention volume, certain hydropeaking effects can be prevented completely or in part - such as large daily variations of water level and flow rates in winter, for example, as well as high hydropeaking rates or fast changes in temperature and turbidity. Other phenomenons, such as the shift in average monthly discharge rates and the comparatively high concentrations of suspended solids in winter still remain, however.

Further research required

As for the river Rhone, several questions on hydropeaking are still not answered for many streams and rivers in the alpine region. Recommended or threshold values that would permit a hydro-ecological assessment of the effects of hydropeaking are still missing to a large extent and too little is known about the interdependencies between hydropeaking and other deficits concerning rivers and streams, in particular on their canalisation.

The Rhone-Thur project

The interdisciplinary Rhone-Thur project launched in 2002 accompanied river engineering projects concerning the rivers Rhone and Thur. The aim of the project was to develop means and tools which could serve as basis for further revitalisation projects and allow the efficient implementation of projects. The project is supported by Eawag, WSL, VAW and EPFL. Further partners are the Swiss Confederation (BWG and BUWAL), the cantons Wallis and Thurgau, the universities of Zurich and Neuchâtel, the Centre for Advice on Water-meadows in Yverdon as well as private environmental and engineering offices. More information on the project can be found under:

More information on hydropeaking can be found under:
Here, you can also find news on river widening projects which can improve habitats on corrected, straightened streams and rivers. Downloads of photos are available under “Media” on

Andri Bryner | alfa
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