Northern Italy has the Alps as northern and western boundary and the Apennine Mountains as the southern one. In between the two mountain ranges lies a large plain made of the Venetian Plain and the valley of the Po, the largest river in Italy, which flows eastward from the Cottian Alps to the Adriatic Sea and receives all the waters that flow from the Apennines northwards, and all those that descend from the Alps towards the south. The Po Valley is the largest plain in Italy and held the vast majority of North Italian population.