river in central Italy, made up of two streams, the Rapido (or Gari) and the Liri, and having a total length of 98 mi (158 km) and a drainage basin of 1,911 sq mi (4,950 sq km). It has its sources near Cappadocia, in the Monti Simbruini east of Rome, and flows south and southeast through a long, narrow, scenic valley in its upper course as far as Arce, where it enters its wide lower valley and receives the waters of the Sacco (left bank) and Melfa (right) rivers. Near San Giorgio it is joined by the Rapido and there becomes the Garigliano River, turning southwest to empty into the Tyrrhenian Sea near Minturno, forming the boundary between Lazio (Latium) and Campania regions in its lower course.

During the winter of 1943–44, as part of the Allied drive toward Rome, heavy fighting took place along the Rapido and Liri, the Allied forces finally crossing in April 1944.