APAVER

Evaluation of the impacts of the partial lowering of the Verbois reservoir on the fish population of the Rhône

The hydroelectric development of the Rhône has greatly disrupted the hydrology and sediment transit. On the Geneva Rhône, the Verbois reservoir is a real trap for fine sediments, which must be regularly «cleaned» in order to guarantee the safety of the lower districts of Geneva (Jonction).

Since 2016, a new fine sediment management strategy has been put in place, jointly with the French authorities and operators (CNR - Compagnie Nationale du Rhône). This strategy is based in particular on controlled flushing operations, where the lowering of the reservoir is partial rather than complete, and where the concentration of suspended solids downstream of the dam must not exceed certain thresholds over a given period. The consequences of this resuspension of fine sediments on the fish fauna are significant.

The objectives of this project are as follows:

• Analysis of fish behavior during the lowering phase of the reservoir;
• Quantification of the mortality induced by the operation;
• Evaluation of the influence of the lowering of the reservoir on fish density;
• Monitoring the resilience of the reservoir population in the medium term.