Seine (sān, Fr. sĕn) [key], Lat. Sequana, river, c.480 mi (770 km) long, rising in the Langres Plateau and flowing generally NW through N France. It passes Troyes, Melun, and Paris, whence it meanders in large loops through Normandy, past Rouen, and empties into the English Channel in an estuary between Le Havre and Honfleur. With its tributaries (the Aube, Marne, Oise, Yonne, Loing, and Eure) and connecting canals, it drains the entire Paris basin. One of the most navigable rivers in France, it has been a great commercial artery since Roman times. The channel of the Seine is dredged and oceangoing vessels can dock at Rouen. Much of France's internal and foreign trade moves on the Seine. Paris, Rouen, and Le Havre owe their prosperity to their favorable location on the river.