Artificial floods mimick natural flows to mobilise and transport sediment downstream, reshape the river and reinstate important habitats for native aquatic insects and fish. Improved outcomes can occur when these artificial floods converge with natural flows and sediment delivery from unregulated tributary streams further down the valley.
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.