Restoring dammed rivers using artificial floods

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An artificial flood on the Spöl River. (Photo: Gabriele Consoli, Eawag)

An artificial flood on the Spöl River. (Photo: Gabriele Consoli, Eawag)

Naturally, water levels of rivers and streams are variable and fluctuate between drier and wetter periods. Spring snowmelt and the timing and location of rainfall events often drive these fluctuations, especially in alpine areas. Sediment and adsorbed nutrients are mobilised by flowing water and transported through river valleys. This, in turn, helps shape the river, its pools and riffles, and connection with the floodplain. Natural stream systems have healthy riverside vegetation and sources of organic matter that form part of the food web ( Robinson, C.T. et al., Freshwater biology, 2002 ), sustaining aquatic insects, fish and other organisms. The hydrology, geomorphology and ecology of natural rivers are in harmony. Dams change all of this.

Dams can change a river’s natural flow

Worldwide, there are more than 55,000 large dams used for the benefit of societies, be it for drinking water supply, irrigation, flood mitigation, or hydro-electric power generation. On the downside, dams can drastically change a river’s natural flow, particularly its variability and the size and timing of floods. This is called ’river regulation’. The reservoirs formed behind dams trap most of the incoming sediment, starving the river and its ecosystems downstream. River channels can narrow or cut deeper. These changes affect riparian vegetation, degrade aquatic habitat and can lead to the collapse of entire food webs or even ecosystems. Adaptive dam management nowadays can use artificial floods ( Robinson, C.T et al. Freshwater Science, 2018 ) to reintroduce some of the elements of natural flow variability into these degraded catchments.

Spöl river catchment as a case study

Little is known about the ecological benefits of these floods as they are not used regularly in many locations. In a recent study, published in Journal of Environmental Management ( Consoli, G. et al., 2022 ), we used the Spöl river catchment as a case study to compare responses to artificial floods upstream and downstream of a free flowing tributary. Results are intended to be translated and applied to restoring other managed rivers, including the role of natural tributaries.

The Spöl catchment arises in the Central Alps and forms part of the Danube basin (Figure 1). Two large dams (Punt da Gall and Ova Spin) were built between 1960 and 1970 for electricity production, regulating the flow of the Spöl. The freely flowing Ova da Cluozza joins the Spöl mid-way between the Ova Spin dam and the river’s junction with the Inn River near Zernez. The Ova da Cluozza’s water levels vary substantially by season, including low winter flows and periodic rainfall or snowmelt-generated floods, reminiscent of the Spöl prior to dam construction.

After the flood, no significant changes in the number (density) and diversity (taxa richness) of macroinvertebrates were recorded downstream of the tributary (sites A & B). However, upstream of the junction (sites C & D), we recorded a significant drop in density and taxa richness - which then recovered after about four weeks. This result confirmed our expectation that above the tributary the floods would have caused greater disturbance to the system. Both floods flushed the river of algae and organic matter on the riverbed, reducing levels at upstream and downstream sites.

Our drone surveys showed that the river’s morphology mostly changed downstream of the tributary after the artificial floods (Figure 4). The channel upstream of the tributary is narrow and shows signs of sediment depletion with vegetated sediment deposits (bars) detached from the stream, as is typical of a regulated river. The downstream section benefits from the free-flowing Ova da Cluozza carrying sediment and debris, which allow for much more dynamic changes in river morphology in response to artificial floods.

Sediments were redistributed and bars in the channel grew larger after the floods. The floods promoted lateral movement of the river and the enhancement of river morphology. The involvement of riparian vegetation resulted in the formation of valuable habitats within the floodplain, as normally seen in rivers unaffected by dams.

Artificial floods are essential to improve dammed rivers

The key takeaway message from our research is that artificial floods are essential to improve dammed rivers by restoring flow variability and physical disturbance. Unregulated tributaries help reduce the impacts of river regulation and enhance the benefits of artificial floods. In such a context, artificial floods are important in mobilising sediment provided by the tributary, which otherwise would accumulate at the confluence. In our future research, we aim to identify the ideal timing, magnitude and frequency of these floods to deliver the best outcomes for stream morphology and ecology, maintaining the societal benefits of dams while minimising impacts on the environment.

Consoli, G.; Haller, R. M.; Döring, M.; Hashemi, S.; Robinson, C. T. (2022) Tributary effects on the ecological responses of a regulated river to experimental floods, Journal of Environmental Management, 303, 114122 (16 pp.), doi: 10.1016/j.jenvman.2021.114122 , Institutional Repository

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