Hydrology

Emmanuel Reynard, Institute of geography and sustainability, FGSE

With the exception of the Doveria Valley, which flows toward Italy from the Simplon Pass, and the northern slopes of the Sanetsch (Sarine) and Gemmi passes, the entire canton of Valais is within the watershed of the Alpine Rhône, covering an area of 5,244 km², from the Rhône Glacier (ranging from 3,600 m to 2,208 m) to Lake Geneva (372 m). Currently, the Rhône originates from a lake formed at the front of the glacier, dammed by a glacial moraine. From the glacier to Lake Geneva, the Rhône covers 164 km, with an average gradient of 0.89%, though with significant variations depending on the sector. The average altitude of the watershed is 2,127 m.

There are several hydrometric stations in the Rhône basin. The oldest is located at the Porte-du-Scex, near the river’s mouth in Lake Geneva, where water levels have been monitored since 1863, and a limnigraph was installed in 1891. By 2015, 16 hydrometric stations were operational in the Alpine Rhône basin, seven on the Rhône and nine on its tributaries. Data is published by the Federal Office for the Environment in the Swiss Hydrological Yearbook.

The Rhône watershed has a high glaciation rate of 11% and includes some of the largest glaciers in the Alps, including the Grosser Aletschgletscher, Fieschergletscher, Gorner, and Corbassière glaciers. The natural hydrological regime of the Rhône is glacio-nival (upstream) to nivo-glacial (downstream); it is characterized by high flow rates in summer and low flow rates in winter. The average annual flow rate at the Porte-du-Scex is 180 m³/s (from 1905 to 2018). The average summer and winter flow rates are 263 m³/s and 97 m³/s, respectively. The commissioning of large hydroelectric facilities (Grande Dixence, Mauvoisin) in the late 1950s significantly altered the annual flow distribution. Before 1958, the average winter flow at the Porte-du-Scex was 72 m³/s, and after this date, it increased to 120 m³/s. This increase in winter flows is further intensified by current climate warming (fewer snow-related precipitation). The volume stored in the reservoir lakes is estimated at 1,195 million m³, which corresponds to 21% of the annual flow at the Porte-du-Scex. The highest daily flow recorded at the Porte-du-Scex was 1,363 m³/s on October 15, 2000.

The hydrological regime of the Rhône’s tributaries is primarily determined by the average altitude of the watershed. Solid storage (snow, ice) is essential and leads to a decrease in flows during winter, with a peak during the melt season. The type of regime is also influenced by the rate of glaciation. The large glaciated watersheds on the left bank (Vispa, Navisence, Borgne, Dranse de Bagnes) have a glacial-type regime, as do the Massa (Aletsch) and the Rhône upstream of Gletsch. In contrast, the valleys on the right bank, the Chablais region, and the smaller watersheds on the left bank (Printse, Réchy Valley) tend to have nival regimes. A few small watersheds (Illbach) have a torrential regime. Most rivers have been developed for hydroelectric production, which affects their regime, with many sections now having residual flows (especially in the side valleys) or a run-of-river regime (especially the Rhône).