Arno, river, c.150 mi (240 km) long, rising in the Northern Apennines, Tuscany, central Italy, and flowing south to Arezzo where it turns northwest; it proceeds generally west, through Florence and Pisa, to empty into the Ligurian Sea. The Arno valley is fertile and densely populated. Its upper valley, the Casentino, is famous for its scenery. In 1966 a great flood on the Arno heavily damaged the art treasures of Florence.

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