Tarn, river, c.235 mi (380 km) long, rising in the Cévennes Mts., S France, and flowing southwest before emptying into the Garonne River. Deep gorges and canyons formed by the river are tourist attractions. Near Millau the Tarn gorge is crossed by the cable-stayed, multispan Millau Viaduct (2004), which soars to 1,125 ft (343 m) at the top of its highest pylon and is the world's highest road bridge.