The Apennines run along the entire length of the peninsula, dividing the waters into two opposite sides. On the other hand, the rivers are numerous due to the relative abundance of rains and to the presence of the Alpine chain in northern Italy with snowfields and glaciers. The fundamental watershed follows the ridge of the Alps and the Apennines and delimits five main slopes, corresponding to the seas into which the rivers flow: the Adriatic, Ionic, Tyrrhenian, Ligurian and Mediterranean sides. Taking into consideration their origin, the Italian rivers can be divided into two main groups: the Alpine-Po rivers and the Apennine-island rivers.