Arno, a river of Tuscany, rises on the S. slope of Monte Falterona in the Apennines, 6 m. N. of Prato Veccbio, flows S. to the neighborhood of Arezzo, where it is joined by the Chianassa and the Chiana, thence N. W. to Pontassieve, where it receives the Sieve, thence follows a westerly course through Florence and Pisa to 7 m. below the latter city, where it flows into the Mediterranean through a channel cut for it in 1603; length 150 m. It is navigable for small vessels from the sea to Florence, but further is liable to be obstructed by floods and droughts. To guard against the former, it has been embanked for the greater part of its course. The valley through which the Arno flows between Florence and Pisa is the very garden of Italy, and is famous for its beauty.