The first aim of this contribution was to find new water resources to provide correctly the developing town of Saint-Flour and the villages on the Planeze, northward. The application of a geophysical method, magnetotellurics, complemented the first geological studies and after three drillings the initially estimated production of 200 m 3 /h was reached. In a second time the authors completed their data especially on volcanic vents and in their immediate surroundings. The more documented maps obtained this way, in particular isoheights, isohypses and isopachs maps lead to the discussion of the evolution of the drainage pattern in relation with volcano-tectonics and glacial events. In conclusion the hypothesis of Ph. Glangeaud  by the terms of which the Truyere river flew into the Alagnon and the Allier (Loire basin) before its capture by the Lot (Garonne basin), was confirmed. They have also contributed in finding an old bed of the Alagnon under the Planeze.