The Aosta Valley coincides with the upper stretch of the Dora Baltea basin, which arises from the slopes of the Mont Blanc massif and flows into the Po after a distance of about 160 km, 100 of which are included in the Valle d’Aosta area.
Its tributaries collect the waters coming from the numerous side valleys. On the orographic left there are: the Doire de Ferret stream, coming from Val Ferret; the Buthier, from Valpelline, which in turn collects the waters of the Buthier d’Ollomont streams, from the Ollomont valley, and Artanavaz (or Buthier d’Artanavaz), from the Gran San Bernardo valley (or Combe Froide); the Saint-Barthélemy stream from the valley of the same name; the Marmore from Valtournenche; the Evançon from the Challant-Ayas valley; the Lys from the Gressoney Valley. On the orographic right they meet: the Doire de Veny, from Val Veny; the Doire de Verney from the Piccolo San Bernardo Valley (or La Thuile Valley, or Valrutor); the Doire de Valgrisenche from Valgrisenche; the Doire de Rhêmes from the Rhêmes Valley; the Savaraz from Valsavarenche; the Grand-Eyviaz from the Cogne Valley; the Clavalité from the homonymous valley; the Ayasse from the Champorcher Valley.
There are numerous lakes of glacial origin, none of which however reaches significant dimensions.