The river Maggia originates in the north-western Alps near the Cristallina mountain and flows through the Sambuco, the Lavizzara, and the Maggia Valley into Lake Maggiore between Ascona and Locarno forming a delta. Located upstream of Ponte Brolla (216 m a.s.l.) in Canton Ticino in Switzerland, the Maggia Valley lies in the southern part of the Swiss Alps and rises up to 3272 m a.s.l. at Mount Basòdino, which contains the only remnant glacier in the catchment. The geology of the catchment is predominantly composed of gneiss and granite.
The climate is Mediterranean with a long-term mean annual rainfall of approximately 1730 mm (Cevio, 1901-2016). Summers are relatively dry and heavy rainfall typically occurs during autumn. Winter precipitation is scarce, occurring mainly as snow due to the high altitude. Precipitation exhibits a considerable spatial variation because of the presence of orographic effects (Ruf, 2007).
The natural streamflow regime of the Maggia River (streamflow gauge in Bignasco) can be described as pluvio-nival with a typical snowmelt peak in June. Streamflow exhibits a high temporal variability due to the rapid catchment response to precipitation. This results from the very steep slopes, the thin soil layers as well as granite and gneiss bedrock. Streamflow varies from less than 1 m3/s under natural low flow conditions up to several hundreds of m3/s during extreme flood events. Largest flood peaks may reach a discharge close to 1000 m3/s (Ruf, 2007).
The main valley, located in the lower part of the catchment (section Bignasco-Ponte Brolla) is characterized by steep rocky slopes, that cause fast and intense runoff events, and a relatively well-defined floodplain. After the last period of glaciation, Maggia valley has been filled by coarse alluvial material forming a 150 m thick layer (Ruf et al., 2008).
The project area is the riverine corridor in the main valley, extending from Bignasco to Ponte Brolla and having approximately a length of 22 km and a width of 500 m. The riverbed is characterized by strong dynamics and high sediment transport rates. As a result, a braided river morphology with gravel bars and vegetated islands is observed over a length of around 7.5 km near the village of Someo in the central part of the corridor.
Historical references report vast inundations and frequent relocations of the channels in river Maggia (Cerini, 2003). The braided area is a very active floodplain with riverine vegetation undergoing natural dynamics of succession and rejuvenation. The braided area has an average slope ranging from 0.5-1%, while in the upper and lower parts channel slopes reach 2%. The entire floodplain in the main valley consists of coarse material (cobbles, large gravels and sand with a pronounced armouring layer).
The Maggia Valley, characterised by the presence of a large hydropower system (OFIMA) with many reservoirs in the upstream part of the catchment, is an example of an alpine environment highly affected by streamflow regulation due to hydropower activities. Its riverine corridor, which contains one of the few remaining natural alluvial river systems with a braided gravel riverbed and a riparian floodplain forest in Switzerland, has undergone a strong reduction in discharge magnitude and variability as well as a drop in groundwater levels and sediment supply. The affected river reach is classified as a wetland of national importance since 1992.
Similarly to many alpine valleys, hydropower system construction in the 1950s led to a dramatic drop in the mean annual streamflow in the affected river reach. This change amounts to about 75% of the natural pre-dam discharge, i.e. from 520 mil. m3 in the pre-dam period to about 130 mil. m3 in the post-dam period. The seasonality of streamflow also changed significantly, with a reduction of the spring-summer snowmelt runoff peak due to filling of reservoirs. The imposition of minimum flow requirements in 1982 led to an almost constant flow for most of the year. However, because the upstream reservoirs are not large in size and are not built for flood protection, the most consistent discharges in the affected reach are not significantly reduced, so geomorphic reworking of the gravel bed by large floods continues to take place.
A view of the main valley with its steep hillslopes (©Wolfgang Ruf)
Human impacts mainly affects both the hydrological regime and sediment budgets.
Concerning the hydrological regime, the low (and constant) streamflow and the drop in groundwater levels are likely to have a significant effect on riparian vegetation. On a long-term perspective there should be a balance between vegetation destruction due to flooding and scour and vegetation regeneration on newly established gravel bar surfaces. A strong tendency towards narrowing of the river and the exposed sediment area in the post-dam period was observed (Ruf, 2007). This general tendency occurs despite the fact that the river system is still able to rework the river bed and to erode pioneering vegetation (mostly Salicaceae) significantly after large flood events (e.g. in 1978). For managers of river systems the question then is whether hydropower operation has a significant effect on floodplain processes due to the altered surface water flow regime or changes in groundwater levels.
Considering the effect on sediment budgets, it is expected that reservoir construction in the upper part of the river may significantly reduce sediment input into the affected river reach and/or shift the size composition of the incoming sediment towards coarse material. This may have significant effects on floodplain soil water storage, the erosion of the river bed and transport of coarse grains.
The TiRiLab which is an environmental computing laboratory at ETH Zurich within the HyMoCARES project will use advanced hydrodynamic modelling in the Maggia to provide a DEMO of options focusses around the fundamental questions: (a) can we quantify the erosion/deposition rates in the river bed as well as the required flood level for a significant geomorphic work to take place?; (b) can we quantify alterations of the physical habitat, transportability of sediment, and habitat suitability for fish with respect to the pre- and post-dam period, with a view towards full ecosystem services analysis?
The braided area of the Maggia Valley (©Wolfgang Ruf)