Po (ancient Padus), river in northern Italy, the longest in the country. Rising in the northwest near the border with France, on Monte Viso in the Cottian Alps, it flows east for a distance of 652 km (about 405 mi), past the cities of Turin, Chivasso, Trino, Piacenza, Cremona, Casalmaggiore, Viadana, Ferrara, and Porto Tolle, into the Adriatic Sea. Its drainage basin covers a large portion of the region. Its tributaries are the Tanaro, Dora Baltea, Dora Riparia, Ticino, Adda, Oglio, Mincio, Trebbia, Taro, Panaro, and Secchia rivers. The surface of the river along its lower course is high, and the land is protected by canals, levees, and dikes. The river is navigable by small craft for 480 km (300 mi), but sluggish flow periodically makes navigation difficult; it is also used for irrigation. Deposits of silt, brought by many tributaries, are enlarging the delta of the Po at the mouth of the river to the extent that the delta is advancing into the Adriatic Sea at the rate of about 60 m (about 200 ft) per year.