Fluctuations in the flow of rivers and streams may have diverse natural or anthropogenic causes. The hydropeaking considered here is caused by the rapid increase or decrease in the release of (operating) water from reservoir hydroelectric power stations when there is a great fluctuation in the demand for power. This method of production changes the flow regime in the rivers downstream of the plant according to the season, in that considerable quantities of water are shifted from summer to winter. But the intermittent turbining also causes short-term, artificial fluctuations in flow, in a weekly and daily rhythm. This frequent and regular occurrence also fundamentally differentiates the change between the high (surge) and low flow produced by the power plant from natural flooding events.

Of the medium-size to large rivers in Switzerland, about one in four is influenced by the return of water from reservoir hydropower stations; the rivers of the Alps and Prealps are particularly affected. In addition to removal or diversion of water (residual flow stretches), and the flushing of hydropower facilities (reservoirs, intakes, equalising reservoirs), hydropeaking is therefore a widespread and severe consequence of the hydroelectric exploitation of rivers.

Hydropeaking-related effects on the Rhone

A further anthropogenic intervention affects the Rhone and many other surge stretches at least as strongly as hydropeaking itself: straightening, narrowing and engineering of the channels as a result of river corrections, which generally took place or were started already before hydroelectric exploitation. With hindsight, i.e. because of today’s situation alone, it is often scarcely possible any longer to distinguish the impacts of morphological and hydrological changes. The Rhone-Thur Project therefore evaluated a few older principles of hydrology, morphology and biology of the Rhone, which permit a better insight into the previous state of the river and its development. Based on this, various indicators were developed, using which the hydropeaking-related changes could be better described and assessed.

The construction and operation of numerous reservoir power stations greatly changed the hydrology of the Rhone, particularly in the period from 1950 to 1975. Of particular ecological significance are the substantially higher flows and stronger fluctuations in the winter half-year, as a constant, low water level would predominate under natural circumstances in this period. A further drastic consequence of hydropeaking is the increasing speed with which the water levels or flow changes. During the winter, these hydropeaking rates in the lower reaches of the Rhone now reach an ecologically relevant to critical level.

Together with the flow, water temperature and turbidity – an important parameter of water quality – also fluctuate in the Rhone under the effect of surges. These fluctuations are hardly great enough to cause acute damage to aquatic organisms. Nevertheless, changes in metabolism or behaviour should be expected, primarily among sensitive animals, and again in winter.

As a consequences of the river correction, and reinforced by some effects of hydropeaking (increased turbidity and stronger vibration of the streambed material in winter), the streambed of the Rhone has become increasingly clogged (internal colmation) compared to the natural state. The transparency of the streambed material and the exchange between Rhone and groundwater has thus also decreased. Ripping up and flushing out the bed (unplugging) does not, however, take place more frequently with today’s morphology, because the necessary flow limits are not reached by far, even during the biggest surges.

The groups of organisms living in and directly by the Rhone that were investigated in more depth do not all react in the same way to hydropeaking. In addition to the differences between the organisms, diverse, sometimes specific factors also contribute to this. For example, some fish species (as all other organisms) are also significantly affected by the river’s morphology and are (the only ones) also subject to extremely artificial stocking. Both in aquatic (fish, macroinvertebrates) and terrestrial organisms (invertebrate riverbank fauna), the overlapping of different influences makes it very difficult to isolate and clearly identify the impacts of hydropeaking.

Despite this often inadequate selectivity, hydropeaking is doubtless one of the main causes of the biological deficits found in the Rhone. For example, the disproportionate washing away of macroinvertebrates when the flow rises (catastrophic drift), and the mobilisation of deposits of fine debris when the flow is high, which severely affects the streambed habitat, are proven to be primarily surge-related. Particularly great deficits have been found in all the organism groups investigated downstream of Riddes, where the effect of surges increases strongly through two large power stations nearby. From there to the outlet into Lac Léman at Porte du Scex the hydrological and biological state of the Rhone did not improve appreciably.

In some stretches of the Rhone itself (e.g. in the Pfynwald – Iles Falcon residual flow stretch), as well as in some tributaries and inlets, some plant and animal species have held their ground, but have otherwise disappeared from large stretches of the main river. The Rhone therefore still has a substantial potential for future recolonisation.

Impacts of hydropeaking on rehabilitation measures

Hydropeaking is as aggravating constraint for the planned rehabilitation of some stretches of the river. This applies particularly to the actual river basin (aquatic part), while the measures around the river (e.g. reactivation of alluvial zones, promotion of terrestrial pioneer vegetation) are less severely affected by short-term fluctuations. In the aquatic part of the Rhone, particularly downstream of Riddes with its severe surges, the success of rehabilitation is however fundamentally disputed, if effective surge mitigation measures are not taken at the same time. But such measures are also indicated upstream, where the effect of surges is less. The present report contains the principles for determining the extent of the effects of surges, and for deciding which mitigation measures would be possible and suitable on a case-by-case basis.

On the other hand, knowledge is still fragmentary concerning how far the effect of surges must be reduced to restore the target ecological quality or functioning of the river. This question must still be clarified for each individual case. The present report provides some aids and criteria for this in the form of possible indicators and benchmarks.

For the Rhone and other surge-affected rivers of the Alps, the investigations up to now of the Rhone-Thur Project have produced a need for further research with the following priority topics:

• The association between morphology (channelisation) and hydrology (hydropeaking) is still insufficiently investigated. For example, how a rehabilitation using river widening changes the impacts of given hydropeaking is still largely unknown. According to the current state of knowledge it cannot be ruled out that rehabilitation measures will adversely change some surge-related parameters – even if the positive impacts on the ecology of the river and its surroundings generally clearly predominate.

• There is still a lack of reference or limit values that would permit an ecological assessment of given hydropeaking (possibly dependent on morphology), although approaches do exist for some surge indicators, such as the flow regime of hydropeaking or for the maximum change rate of the water level. But these suggestions have not been thoroughly tested for their suitability for Swiss surge stretches with varying morphology.

• The impacts of hydropeaking on the colmation of the streambed have been calculated and modelled in some cases, but hardly ever in situ, i.e. reliably recorded in the rivers themselves. Suitable measuring methods would probably need to be developed and tested for this.

• The efficacy of different surge mitigation measures on the river ecology has been investigated in only a few examples. Further assessments of the outcome of measures already realised are also needed to create better foundations in this field.

Citation:
Baumann, P., Meile, T., Fette, M., 2005: Synthesebericht Schwall-Sunk. Eawag, WSL, LCH-EPFL, VAW-ETHZ. 48 pp.