Po (anc. Padus and Eridanus), a river of N. Italy, having its source in Piedmont, in two springs about 6,000 ft. above the sea, near lat. 44° 40' N, Ion. 7° E., on the E. side of Monte Viso, one of the Cottian Alps. It flows E. in a winding and irregular course for about 450 m. across the whole breadth of northern Italy, and enters the Adriatic by a delta, the most important branches of which are the Po della Maestra, Po di Volano, and Po di Primaro, between lat. 44° 35' and 45° N, and Ion. 11° 55' and 12° 30' E. It receives a great number of tributaries both from the Alps and the Apennines. The most important of the former are the Dora Kiparia, Clusone, Sangone, Stura, Dora Baltea, Sesia, Tanaro (from the south), Ticino, Olona, Adda, Oglio, and Mincio; and of the latter, the Scrivia, Trebbia, Nura, Ta-ro, Parma, Enza, Secchia, Panaro, and Reno. About 50 towns of considerable size are situated on its banks or those of its tributaries. Boats can ascend to within 60 m. of its source, but the current is so rapid as to render navigation difficult. Destructive floods are liable to happen at all seasons, and the flat country along its lower course renders artificial embankments necessary.

Below Piacenza dikes have been formed, but extensive inundations occasionally happen. The bed of the Po is continually raised by its deposits, and this renders necessary a corresponding increase in the embankments, so that in many places the surface of the river is from 15 to 20 ft. higher than the adjacent country. The breadth of the Po from the Ticino to the delta varies from 400 to 600 yards, and the depth from 12 to 36 ft. Its basin includes an area of about 40,000 sq. m., and comprises the whole of Piedmont and Lombardy, parts of southern or Italian Tyrol, western Venetia, and the Swiss canton of Ticino, a part of the canton of Grisons, the former duchies of Parma and Modena, the territories of Bologna, Ferrara, and Ravenna, and a small part of Tuscany. Fish abound, including salmon, shad, and sturgeon.