File:Arno watershed.png|thumb|Map of the Arno River watershed (Interactive map) The river originates on
Monte 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
Ligurian Sea at
Marina di Pisa. With a length of , it is the largest river in the region. It has many tributaries:
Sieve at long,
Bisenzio at ,
Ombrone Pistoiese at , and the
Era,
Elsa,
Pesa, and
Pescia. The
drainage basin amounts to more than and drains the waters of the following subbasins: • The
Casentino, in the
province of Arezzo, formed by the upper course of the river until its confluence with the Maestro della Chiana channel. • The
Val di Chiana, a plain drained in the 18th century, which until then had been a marshy area tributary of the Tiber. • The upper
Valdarno, a long valley bordered on the east by the
Pratomagno massif and on the west by the hills around
Siena. • The Sieve's basin, which flows into the Arno immediately before Florence. • The middle Valdarno, with the plain including Florence,
Sesto Fiorentino,
Prato, and
Pistoia. • The lower Valdarno, with the valley of important tributaries such as the Pesa, Elsa, and Era and in which, after
Pontedera, the Arno flows into the
Ligurian Sea. The river has a very variable discharge, ranging from about to more than . The mouth of the river was once near
Pisa but is now several kilometres westwards. over the Arno in Florence It crosses
Florence, where it passes below the
Ponte Vecchio and the
Santa Trinita bridge (built by
Bartolomeo Ammannati but inspired by
Michelangelo). The river
flooded this city regularly in historical times, most recently in
1966, with after rainfall of in Badia Agnano and in Florence, in only 24 hours. Before Pisa, the Arno is crossed by the Imperial Canal at La Botte. This water channel passes under the Arno through a tunnel, and serves to drain the former area of the
Lago di Bientina, which was once the largest lake in
Tuscany before its reclamation. The flow rate of the Arno is irregular. It is sometimes described as having a torrentlike 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 . New
dams built upstream of Florence have greatly alleviated the problem in recent years. s of Arno river floods on August 13, 1547 (left) and November 3, 1844 (metal plate on the right). Photographed in Via delle Casine. The
flood on November 4, 1966 collapsed the embankment in Florence, killing at least 41 people and damaging or destroying millions of works of art and rare books. New conservation techniques were inspired by the disaster, but even decades later hundreds of works still await restoration. == Etymology ==