The river originates on Mount Falterona in the Casentino area of the Apennines, and initially takes a southward curve. The river turns to the west near Arezzo passing through Florence, Empoli and Pisa, flowing into the Tyrrhenian Sea at Marina di Pisa.
With a length of 241 kilometres (150 mi), it is the largest river in the region. It has many tributaries: Sieve (60 kilometres (37 mi)), Bisenzio (49 kilometres (30 mi)), Era, Elsa, Pesa and Pescia.
It crosses Florence, where it passes below the Ponte Vecchio and the Santa Trìnita bridge (built by Bartolomeo Ammanati, but inspired by Michelangelo). The river flooded this city regularly in historical times, the last occasion being the famous flood of 1966, with 4,500 m³/s after rainfall of 437.2 millimetres (17.21 in) in Badia Agnano and 190 millimetres (7.5 in) in Florence, in only 24 hours.
The flow rate of the Arno is irregular. It is sometimes described as having a torrent-like behaviour, because it can easily go from almost dry to near-flood in a few days. At the point where the Arno leaves the Apennines, flow measurements can vary between 0.56 m³/s and 3,540 m³/s. New dams built upstream of Florence have greatly alleviated the problem in recent years.
A flood on November 4, 1966 collapsed the embankment in Florence, killing at least 40 people and damaging or destroying millions of works of art and rare books. New conservation techniques were inspired by the disaster, but even 40 years later hundreds of works still await restoration.