The stream Pesio, which gives the name to the entire Valley, collects the waters of the Pis, Saut and Marguareis valleys. The stream Pis del Pesio, forms a jump of about 30 m: it gushes directly from the rock collecting the waters of the overhanging Conca delle Carsene.

Under the Pian delle Gorre the stream receives the waters from the Rio del Saut and from some sub-river-beds sources. It proceeds then towards the Certosa and from here, after having acquired the waters of some streams, towards Chiusa di Pesio. It receives the waters of its main tributary the Brobbio near Morozzo and continues its route till Carrù where it flows into the river Tanaro.  

The Pesio, crossing the entire Valley allowed the development of many production activities in the past: the people from the Valley used the stream to transport timber, while the first industries used the abundant waters. Nowadays the upper river course is destination for sports' people and fishermen.