The Hermance is a river in France and Switzerland, that discharges into Lake Geneva. It rises near Loisin in Haute-Savoie, France, at 563 m elevation. In its lower half, downstream from Veigy, it marks the Swiss-French border. It flows into Lake Geneva between the villages Hermance (Switzerland) and Chens-sur-Léman (France), at 372 m elevation. Its total length is 14 km (13 km according to the Sandre database), of which 6.5 kilometres on the border. Its catchment area is 42.5 km2, of which 37 km2 in France. Flood control and other river maintenance issues are regulated under the 1959 Hermance River Agreement.

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