Author(s): Charlotte Girard; Franck Pressiat; Marion Cherrak; Seydina Diouf; Laurent Pinet

Linked Author(s):

Keywords: International governance; Sediment management; Participative process

Abstract: The Rhône River, from its glacial source to the Mediterranean, has undergone extensive anthropogenic modifications since the 19th century, notably through hydroelectric developments that have significantly modified its hydraulic regime. In the Geneva section, more than a century of river engineering—gravel pits, dams, and other interventions—has compounded the impact of substantial sediment inputs from the Arve River: between 10,000 and 200,000 m³/year of bedload depending on hydrological conditions, and approximately 700,000 m³/year in suspension. These deposits result in an average accumulation of 360,000 m³/year in the Verbois reservoir, located 14 km downstream from Lake Geneva and 12 km from the Rhône– Arve confluence. This sedimentation can raise water levels and groundwater tables, posing flood risks for lower Geneva's district. Historically, sediment management relied on flushing operations (1945–2012). However, growing environmental awareness prompted a paradigm shift, leading to the establishment of a Franco–Swiss working group tasked with developing alternative strategies. In 2014, this collaboration resulted in a management scenario comprising the following components: • Partial drawdowns of the Verbois (CH) and Génissiat (FR) reservoirs, combined with a full drawdown of the intermediate Chancy–Pougny reservoir (CH & FR), conducted in May/June under strict monitoring to maintain suspended sediment concentrations below ecological thresholds defined by French authorities—classified as Controlled Sediment Flushing Operations (CSFO). • Flood support from the Arve, involving increased discharge at Verbois through releases from Lake Geneva to dilute suspended sediments and enhance transport. • Targeted dredging to ensure flood safety for Geneva. • Annual biodiversity monitoring (fish, birds, macroinvertebrates, mammals, insects). This approach was formalized in an action plan authorized by Swiss and French authorities for 2016 –2026 following a comprehensive environmental impact assessment. As this authorization nears its end, a new permit must be secured. This marks the beginning of a renewed phase of Franco–Swiss collaboration aimed at refining existing transboundary management methods and exploring innovative solutions. Current challenges include the gradual return of Arve gravel to the Rhône in Geneva, the need for improved sediment transparency in Upper Rhône structures, heightened safety concerns in social and political spheres, hydrological and sedimentary shifts driven by climate change, and the ongoing decline of alluvial biodiversity. This paper outlines the methodological framework guiding operators in addressing these challenges through a participatory, transboundary approach designed to foster stakeholder acceptance of technical solutions.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.283

Year: 2026