The Seine is a 776 km (482 mi)-long river and an important commercial waterway within the Paris Basin in the north of France. It rises at Source-Seine, 30 kilometres (19 mi) northwest of Dijon in northeastern France in the Langres plateau, flowing through Paris and into the English Channel at Le Havre (and Honfleur on the left bank). It is navigable by ocean-going vessels as far as Rouen, 120 km (75 mi) from the sea. Over 60% of its length, as far as Burgundy, is negotiable by commercial riverboats and nearly its whole length is available for recreational boating; excursion boats offer sightseeing Tours of the Rive Droite and Rive Gauche within the city of Paris.
There are 37 bridges within Paris and dozens more spanning the river outside the city. Examples in Paris include the Pont Louis-Philippe and Pont Neuf, the latter of which dates back to 1607. Outside the city, examples include the Pont de Normandie, one of the longest cable-stayed bridges in the World, which links Le Havre to Honfleur.
The Seine is dredged and oceangoing vessels can dock at Rouen, 120 km (75 mi) from the sea. Commercial riverboats can use the river from Bar-sur-Seine, 560 km (350 mi) to its mouth. At Paris, there are 37 bridges. The river is only 24 metres (80 ft) above sea level 446 km (277 mi) from its mouth, making it slow flowing and thus easily navigable. The Seine Maritime, 105.7 km (65.7 mi) from the English Channel at Le Havre to Rouen, is the only portion of the Seine used by ocean going craft. The tidal section of the Seine Maritime is followed by a canalized section with four large multiple locks until the mouth of the Oise River at Conflans-Sainte-Honorine.
Multiple locks at Bougival / Chatou and at Suresnes lift the vessels to the level of the river in Paris, where the mouth of the Marne River is located. Upstream from Paris seven locks ensure navigation to Saint Mammès, where the Loing mouth is situated. Through an eighth lock the river Yonne is reached at Montereau-Fault-Yonne. From the mouth of the Yonne, larger ships can continue upstream to Nogent-sur-Seine. From there on, the river is navigable only by small craft. All navigation ends abruptly at Marcilly-sur-Seine, where the ancient Canal de la Haute-Seine used to allow vessels to continue all the way to Troyes. This canal has been abandoned for many years.
The basin area is 78,910 square kilometres, 2% of which is forest and 78% cultivated land. In addition to Paris, three other cities with a population over 100,000 are in the Seine watershed-Le Havre, Rouen, and Rheims-with an annual urban growth rate of 0.2%. The population density is 201 per square kilometre.
Periodically the sewerage systems of Paris experience a failure known as sanitary sewer overflow, often in periods of high rainfall. Under these conditions untreated sewage has been discharged into the Seine. The resulting oxygen deficit is principally caused by allochthonous bacteria larger than one micrometre in size. The specific activity of these sewage bacteria is typically three to four times greater than that of the autochthonous (background) bacterial population. Heavy metal concentrations in the Seine are relatively high. The pH level of the Seine at Pont Neuf has been measured to be 8.46. Despite this, the water quality has still improved, with several historians having called it at various times in the past as an "open sewer".
Length: 776 km (482 mi)
Basin area: 78,650 km2 (30,370 sq mi)