Far removed from natural conditions
An exemplary case is the Maggia in Ticino, a little gem of national importance. However, the idyll is deceptive. This is because the lion’s share of the natural discharge from this mountain river’s catchment area is redirected into a system of reservoirs and passes through turbines and pressure pipes rather than picturesque alluvial landscapes.
Only the residual water volume required by law remains in the riverbed – apart from the contribution of small tributaries that are not utilised for hydropower. In the event of severe storms, however, it can also be the case that major discharge peaks are diverted into the river to ensure that the infrastructures used for hydropower are not overloaded. In the river, a gap exists between these extremes. In natural rivers, smaller floods continuously reshape the riverbed and also create connections, for example, across the course of a river. This gives rise to a large number of niches for various fauna and flora. The naturally occurring floods are also important for recharging groundwater reserves – the heightened water level brings about an exchange between the surface water and groundwater.
The researchers from ETH Zurich wanted to obtain a better understanding of how the absence of natural flow dynamics impacts the habitat of the Maggia and similar rivers. Their computer simulations revealed that a simple increase in current residual flow rates could absolutely lead to a slight improvement in environmentally relevant variables. For example, the decline in the groundwater level would not be as marked, while the river would run quicker and achieve greater water depths. Nevertheless, the achieved values would still fall far short of creating the characteristics of a free-flowing natural river.
The species composition found on the Maggia is also far removed from what would be expected under natural conditions, as shown by another study conducted by the EAWAG team. The researchers investigated the life in pools left in the riverbed by a flood. If such pools continue to exist detached from the river flow over an extended period, they increasingly become inhabited by flies, mosquitoes and beetles as time passes, i.e. by insect species that love standing waters. In contrast, young pools are primarily home to stoneflies, mayflies and caddisflies, in other words species that are actually typical of a fast-flowing, cold and oxygen-rich river such as the Maggia. Periodically occurring floods flush out older pools once more and reset the environmental clock. Fluctuations in the water level are therefore necessary in order to preserve habitat diversity.