River Loing. The Loing is a 142 km long river in central France, a left tributary of the Seine. Its source is in Sainte-Colombe-sur-Loing, in the south of the Yonne department, and it flows into the Seine in Saint-Mammès, near Moret-sur-Loing.Its main tributaries are the Ouanne, the Aveyron, the Puiseaux, the Solin and the Lunain. The part of the Briare Canal between Rogny-les-Sept-Écluses and Montargis runs parallel to the Loing.