With a length of 241 kilometres (150 mi), it is the largest river in the region. It has many tributaries: Sieve at 60 kilometres (37 mi) long, Bisenzio at 49 kilometres (30 mi), Ombrone Pistoiese at 47 kilometres (29 mi), and the Era, Elsa, Pesa, and Pescia. The drainage basin amounts to more than 8,200 square kilometres (3,200 sq mi) 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 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 6 cubic metres per second (210 cu ft/s) to more than 2,000 cubic metres per second (71,000 cu ft/s). The mouth of the river was once near Pisa but is now several kilometres westwards.
"Ponte Vecchio" (The old bridge) over the Arno in FlorenceView of the Arno from the Ponte Vecchio
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 4,500 cubic metres per second (160,000 cu ft/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.[citation needed]
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 0.56 and 4,100 cubic metres per second (20 and 144,790 cu ft/s). New dams built upstream of Florence have greatly alleviated the problem in recent years.[citation needed]
High water marks 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 40 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.[9]
From Latin Arnus (Pliny, Natural History 3.50). The philologist Hans Krahe related this toponym on a paleo-European basis *Ar-n-, derived from the Proto-Indo-European root *er-, "flow, move".[10]
Water from the Arno drainage basin is used for drinking water, irrigation, and firefighting.[15] Citizens in the central part of the drainage basin also identified flood control, support for biodiversity, fisheries, and cultural value as other services that the river provides.[15] There is the risk that flooding will jeopardize these ecosystem services, as 9% of wastewater treatment plants, 10% of landfills or other waste sites, and 4.5% of contaminated sites are at high risk of flooding, which would produce hotspots of pollution.[16]
^Parco Nazionale delle Foreste Casentinesi (1999-01-01). Environmental restoration of the area of the river Arno offsprings (Report). European Climate, Infrastructure and Environment Executive Agency. Retrieved 2024-02-16. "...on the southern slopes of Mount Falterona... the springs of the Arno River, described by the great poet Dante Alighieri, and the Etruscan archaeological site Lago degli Idoli are both located here.
^"Arno River". Encyclopedia Britannica. July 20, 1998. Retrieved February 16, 2024.
