Po (pō, It. pô) [key], Latin Padus, longest river of Italy, c.405 mi (650 km) long, rising in the Cottian Alps of Piedmont, NW Italy. It winds generally east in a wide valley, past Turin, Pavia, Piacenza, Cremona, and Ferrara, to enter the Adriatic Sea through several mouths. Its marshy delta is constantly expanding eastward. The Dora Baltea, Tanaro, Ticino, Adda, and Oglio rivers are its chief tributaries; hydroelectricity is produced there. The Po River is navigable for small craft c.300 mi (480 km) upstream, but seasonal variations in flow hamper navigation. It is extensively used for irrigation. The Po valley is densely populated and is the most important industrial and agricultural region of Italy. Grains, sugar beets, livestock, and fruits are raised. Turin, Asti, Milan, Brescia, and Verona are the chief cities of the Po valley.