Ar'no, next to the Tiber the most important river of Central Italy, rises on Mount Falterona, at a height of 4,444 feet above the sea. It flows westward 140 miles and empties into the sea, eleven miles below Pisa. At Florence it is 400 feet wide, but can be forded in summer; at other times it can be navigated by barges thus far. It is noted for its rapid and destructive floods.