The drainage basin amounts to more than 8,200 square kilometres (3,200 sq mi) and drains the waters of the following subbasins: It crosses Florence, where it passes below the Ponte Vecchio and the Santa Trinita bridge (built by Bartolomeo Ammannati but inspired by Michelangelo).

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.

[citation needed] 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.

The philologist Hans Krahe related this toponym on a paleo-European basis *Ar-n-, derived from the Proto-Indo-European root *er-, "flow, move".

[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.