The altitude of the valley through which the Po flows, exclusive of its tributaries, varies from approximately below sea level in the Polesine subregion (the delta around Ferrara) to about at the river's origin in the southern Piedmontese province of Cuneo, also known as the Provincia granda. The valley is crossed by a number of affluents running down from the Alps in the north and from the Apennines in the south. The Po's major affluents include the Tanaro, Scrivia, Trebbia, Panaro and Secchia in the south, Dora Riparia, Dora Baltea, Sesia, Ticino (draining Lake Maggiore), Lambro, Adda (draining Lake Como), Oglio (draining Lake Iseo) and Mincio (draining Lake Garda and called Sarca in its upper reaches) in the north.