There are four major reservoirs upstream of Paris, on the Seine, Aube, Marne and Yonne rivers. These reservoirs partially prevent flooding in winter and spring, and supply water in summer to maintain a minimum river flow. The quantitative and qualitative impact of summer water releases from dams on temperature and quality of downstream rivers has been assessed, thanks to a measurement survey conducted in 1990 and 1991 and use of simulation models.

Two types of reservoirs are distinguished:

– the Yonne reservoir, directly on the river, which is rather deep and can have a significant influence on the near downstream temperature, but hardly any just upstream of the Seine confluence

– the Seine, Aube and Marne reservoirs, bypassing the rivers, which are shallow and have a very moderate impact on downstream temperatures.

The study of an experimental water release on the Seine showed that for this second type of reservoir it is difficult to achieve considerable improvement in water quality in the downstream rivers by means of reservoir releases in summer, except locally in cases of significant pollution inputs. Among the ten parameters under study, conductivity and alkalinity were the only ones measured whose values could be affected by the dam release over a distance greater than 100 km downstream.