FUNCTIONING OF A FRENCH HYDROELECTRIC RESERVOIR - ISOTOPIC APPROACH

In order to better understand the hydric behaviour of the Bort-les-Org ues reservoir, physico-chemical studies, as well as isotopic studies w ere carried out in 1996 and 1997. The physical results show that the l ake is monomictic, with thermic stratification from June to September- October. The high values of pH in the surface water during summer reve al an active development of biological production starting in June. Is otopic values of O-18 show evaporation at the water-atmosphere interfa ce. The C-13 Of the dissolved total inorganic carbon is governed both by atmospheric exchange and biologic activity. Information about the i nternal dynamics of the reservoir such as a water current, an upstream water column renewal (by 'piston effect'), and a downstream sediment resuspension can thus be obtained. ((C) Academie des sciences /Elsevie r, Paris.).