EXPERIMENTAL RESERVOIR WATER RELEASE - IMPACT ON THE WATER-QUALITY ON A RIVER 60 KM DOWNSTREAM (UPPER SEINE RIVER, FRANCE)

The impact of a reservoir water release on water quality of the upper Seine river (France) was studied at the end of the summer of 1990. The flow from a storage reservoir was increased from 6 to 26 m3 s-1 within 8 h for the purpose of the experiment, the river flow upstream from the reservoir being 4 m3 s-1. During the passage of the wave front and until the dilution by the reservoir water, water samples were taken at a high frequency for physico-chemical (temperature, dissolved oxygen, nutrients) and organic matter parameters (dissolved and particulate organic matter, phytoplankton and bacterioplankton). Due to the resuspension of sediments, the wave front caused a strong deoxygenation and an increase in nutrients, dissolved and particulate organic matter concentrations, as well as in the abundance and biomass of the organisms. Whereas all dissolved elements rapidly decreased after the passage of the wave front, particulate organic matter still increased. A trend towards a restoration of the oxygen diurnal cycle of the river was also observed, due to the photosynthetic activity; however, the initial oxygen level was not found because of high turbidity. Algal populations were dominated by periphytic organisms at the wave front passage and then replaced by phytoplanktonic ones before the arrival of the reservoir water; this suggests that particulate organic matter was kept at a high level by the contribution of the river hydraulic annexes (dead arms, gravel pits, etc.), flooded by the increase of the water level. Finally, a dilution by reservoir waters of better quality occurred. The impact of the water release is discussed in terms of ecological functioning of a river system.