The Tarn River (from the Latin tarnis meaning rapid or walled in) is a 375 kilometre (235 miles) long tributary river of the Garonne, and flows through the départements of Lozère (Languedoc-Roussillon région), Aveyron, and then the eponymous Tarn and Tarn-et-Garonne départements, the last three départements being located in the Midi-Pyrénées région in southwest France.
The Tarn River runs in a roughly westerly direction, from its source at an altitude of 1,550 m on Mont Lozère in the Cévennes mountains (part of the Massif Central), through the deep gorges and canyons of the Gorges du Tarn, to Moissac in Tarn-et-Garonne, where it joins the Garonne River 4 km.(2.5 miles) downstream the center of town.
Its basin covers approximately 12,000 km², and it has a mean flow of approximately 140 m³ per second.
The Tarn passes through the towns of Millau, Albi, Montauban and Moissac. The tributaries of the Tarn include the Tarnon, the Dourbie, the Agoût, and the Aveyron, and the Tarn separates the Narbonne and Aquitaine basins.
The Millau Viaduct, the highest bridge in the world, carrying the A75 autoroute across the Tarn Gorge near Millau, opened in December 2004.
The Tarn River is famous for its brutal floodings, which are the most dangerous in Europe along with the Danube River. The famous flooding of March 1930 saw the Tarn rise more than 17 meters (56 ft) above its normal level in Montauban in just 24 hours, with a discharge of 7,000 m³/s (average discharge of Rhine River is 2,200 m³/s; average discharge of Nile River during the traditional annual flooding before the building of the Aswan Dam was 8,500 m³/s; average discharge of the Mississippi River is 16,200 m³/s). One third of the Tarn-et-Garonne département was flooded, about 300 people died, thousands of houses were destroyed, the low districts of Montauban were destroyed, the town of Moissac was almost entirely destroyed.