The first aim of this contribution was to find new water resources to provi
de correctly the developping town of Saint-Flour and the villages on the Pl
aneze, northward. The application of a geophysical method, magneto-telluric
s, 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 part
icular isoheights, isohypses and isopachs maps lead to the discussion of th
e evolution of the drainage pattern in relation with volcano-tectonics and
glacial events. In conclusion the hypothesis of Ph. Glangeaud [1922] by the
terms of which the Truyere river flew into the Alagnon and the Allier (Loi
re basin) before its capture by the Lot (Garonne basin), was confirmed. The
y have also contributed in finding an old bed of the Alagnon under the Plan
eze.