Traditionally the course of the River Arno in Tuscany is divided into six basins which are in fact well enough defined to be useful:
● The Casentino valley , in the province of Arezzo, is formed by the upper course of the river until the confluence with the Maestro della Chiana channel. The Arno arises on Monte Falterona (1654 m) in the extreme north of this basin which is bordered to the east by the Pratomagno massif and to the east by the Tuscan-Emilian Apennines which also form the political boundary between Tuscany and Emilia-Romagna. A bit further south, the Alpe di Catenaia sepate the Valle Casentino from the source of the Tiber. In this basin the Arno descends about 1000 m so that the river and all of its tributaries are essentially torrents.
● The Piana di Arezzo and the Val di Chiana was, in ancient times, a tributary of the Tiber. However, this marshy area was drained, beginning in the 18 C with the construction of the Canale Maestro della Chiana by Ferdinando III of Lorraine and his son Leopoldo II which effectively reversed the direction of flow of the water. This basin is bordered to the north by the Pratomagno massif, to the north west by the Chianti hills and to the east by a series of heights that separate it from the Upper Tiber valley (Alpe di Poti, 974 m, Monte Coreta, 742 m, Alta S. Egidio, 1056 m).
● The Upper Valdarno, a long valley bordered from East by the Pratomagno massif and from West by the hills around Siena.
● The Basin of the Sieve, which flows into the Arno immediately before Florence.
● The Middle Valdarno, with the plain including Florence, Sesto Fiorentino, Prato and Pistoia.
● The Lower Valdarno, a valley receiving important tributaries such as the Pesa, Elsa and Era, and via which, after Pontedera, the Arno flows into the Ligurian Sea. The river has a very variable discharge, ranging from minimum values such as 6 m³/s to more than 2,000 m³/s. The mouth of the river was once near Pisa , but is now several kilometers westwards.
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