ARNO (anc. Arnus), river, Italy, rising in Mte. Falterona, about 25m. E.N.E. of Florence, 4,265ft. above sea. It first runs south-south-east through the beautiful Casentino ; near Arezzo it turns west, and at Montevarchi north-north-west ; sum. below it forces a way through limestone rock at Incisa and iom. farther on it is joined by the Sieve. Thence it runs west to Florence and through the gorge of Golfolina to Empoli and Pisa, and falls into the sea 71m. west of Pisa, after a course of 155m. The Serchio (anc. Auser), joining the Arno at Pisa in ancient times, now flows into the sea independently. Barges can go up the Arno to Florence ; but it is liable to sudden floods, and so needs careful regulation. Great floods occurred in 1537 and 1740; in 1537 the water rose to 8f t. in the streets of Florence. The valley between Montevarchi and Rignano contains fossil bones of deer, elephant, rhinoceros, mastodon, hippopotamus, bear, tiger, etc., of the middle and upper Pliocene ; while S. Giovanni Valdarno, 4m. below the former, is the centre of lignite mines.