All the waters flowing on the region end up in the Dranse river and the Rhone. After Dom, Weisshorn, it is the highest massif of the Alps situated out of the main chain. South of the Grand Combin, the ridge separating the glaciers of Mont Durand and Sonadon reaches the Grande Tête de By a few kilometres away, which is located on the main watershed. The ridge diverges to the south-west and appears to be continuous with the range of the Aiguilles Vertes, or Aiguilles de Valsorey, and that of Mont Vélan. From this branches the lower range, which divides the channel of the Glacier du Mont Durand (north) from the Val d'Ollomont in the Aosta Valley (south), and extends by the Col de Fenêtre to the Mont Gelé.