This measure is intended to reduce the risk of flooding, particularly in Geneva city centre, and to guarantee safety during floods of the Rhône or its tributary, the Arve. According to SIG, the energy supplier of Geneva, around 400,000 cubic metres of sediment carried by the Arve River is deposited in the reservoir of the Verbois dam every year. To remove these sediments an prevent flood risks,   the Verbois and Chancy-Pougny dams are lowered every three or four years.

Three-stage process

1. The water level of the reservoirs is lowered throughout the chain of dams between Geneva and Lyon. This starts with Génissiat in France, then Chancy-Pougny and finally Verbois.
2. The speed of the water increases, allowing sediment to move naturally downstream.
3. Once the sediment has been evacuated, the water level is raised at the various dams.

The partial lowering temporarily accentuates a natural phenomenon, as the Rhône transports sediment downstream every day.

How wildlife and environment are protected

Throughout the lowering operation, the level of suspended solids is monitored and measured at three stations installed at the Pougny bridge, the La Plaine bridge and downstream of the Verbois dam. Strict regulations require an average suspended solids level of 5 g/l to be maintained over the duration of the operation. This is to control damage to the river’s biodiversity, particularly for fish. Other physico-chemical parameters such as pH, conductivity, temperature, dissolved oxygen and ammoniacal nitrogen (N-NH₄) are also monitored to ensure that the concentrations reached are not toxic for the environment.

Fish populations are closely monitored throughout the partial lowering of the river using techniques such as echo sounding and telemetry. At the Verbois and Chancy-Pougny fish passes, fish are captured, equipped with transmitters, and then released back into the Rhône, allowing researchers to track their movements.

Additionally, rescue fishing operations are conducted to recover fish that become stranded in areas cut off from the main river as water levels drop. To further protect aquatic life, a dedicated refuge zone has been established at the mouth of the Allondon, a tributary of the Rhône.

Beavers are watched and fed by placing food cases. Birds are inventurized before and after the operation. Finally, the impact of the emptying procedure to the reed beds is monitored.

Coordination and players involved

Nearly 400 people are involved in this operation, on both the Swiss and French sides, lémonbleu tv informed about.

The main players are: the Compagnie Nationale du Rhône (CNR), the Services Industriels de Genève (SIG), the operators of the Génissiat, Seyssel, Sault-Brénaz, Chancy-Pougny and Verbois dams, and the administrative and environmental authorities in both countries.