Résumé : La vallée de l'Isère est un champ d'investigation privilégié pour l'étude d'un hydrosystème perturbé par les effets cumulés des interventions humaines. Les aménagements hydroélectriques qui se sont multipliés dans la seconde moitié du XXe siècle dans les hauts bassins ont un impact majeur sur l'hydrologie de l'Isère et ses affluents. Grâce aux séries de mesures hydrométriques disponibles, les perturbations sont quantifiables à tous les pas de temps. De cette nouvelle hydrologie dépendent en partie les fluctuations des lignes d'eau dans le lit ainsi que d'autres changements dans l'hydro-système (niveau de la nappe, morphologie...).

Summary. — Despite the construction of retaining walls designed to protect the people living alongside the river Isère from rising waters, the town of Grenoble is still threatened by floods. Although for more than a century the river Isère has not known big floods (about 1 500 to 2 000 ms/s), such a possibility of flooding should not be dismissed. A calculation of the frequency of such floods-would allow that the maximum flow of 1 500 m'/s constitutes a centenary flood; now, on last September 23, the Isère rolled 1 000 m'/s at Grenoble, and overflowed, as foreseen, at certain weak points in the wall embankment. Therfore, il would seem vital to insure an efficient protection of that town.

The 1976 Spring drought and heat had hydrological consequences in the Northern Alps which are interesting to be studied because of their variety. The deficiency of the pluviométrie totals from december 1975 to june 1976 (the lowest ever recorded) caused the lowest wafer levels ever observed in may average monthly discharge and june for pluvial supplied streams = 0,20). 1976 monthly discharge As the snow cover was thinner than usual because of less important snow falls, the deficiency of the discharge for the snow supplied streams was rather high (0,50 to 0,60 of the usual monthly discharge). The peak value of the monthly average flows occurred in may and not in june due to the early heavy heat. This one and this exceptional daily insolation from may 1st to june 24th (50 % days had more than 11 hours of sun) caused surplus discharges in may (1,20 to 1,68 of the normal value) for high altitude streams (having more than 20 % of glacier area in the river-bassin) .

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