The Orge is a 53-kilometre (33 mi) long river in France, left tributary of the Seine. Its source is in the village Saint-Martin-de-Bréthencourt. Its course crosses the départements of Yvelines and Essonne. It flows northwest through the towns of Dourdan, Saint-Chéron, Breuillet, Arpajon, and Savigny-sur-Orge, finally flowing into the Seine in Athis-Mons, south of Paris. The towns located on the banks of the river usually have s/ Orge added to their name (i.e.