Many dams are built for a primary purpose but, very often, they can fulfill several roles and sometimes simultaneously. A dam intended for hydroelectric production is also a place of tourist activity but also, thanks to the large quantity of water stored, can contribute to low-flow support. One third of dams more than 15 metres high combine several uses.
The large reservoirs upstream of Paris (for example the Pannecière dam) retain part of the flood volume in winter (flood attenuation function) and, conversely, during the hottest months contribute to maintaining a sufficient flow in the Seine at the entry into the Paris region. Located upstream of the Durance chain, the Serre-Ponçon hydroelectric dam also plays an essential role in the irrigation of the entire area downstream of this major development (150,000 ha irrigated), the drinking water supply of numerous municipalities including Sisteron and Marseille, not to mention the tourist asset it brings to the entire region.
Contractual agreements between the owner of the structure and other interested parties define the volumes of water, periods of use, etc.
It should be noted however that at any given moment, certain uses may be incompatible; for example hydroelectric energy production in winter is optimal when the reservoir is full, whereas the flood attenuation function would require an empty reservoir.