The classification of a reservoir by volume is not as straightforward as it may seem. As the name implies, water is held in reserve by a reservoir so it can serve a purpose. For example, in Thailand, reservoirs tend to store water from the wet season to prevent flooding, then release it during the dry season for farmers to grow rice. For this type of reservoir, almost the entire volume of the reservoir functions for the purpose it was built. Hydroelectric power generation, on the other hand, requires many dams to build up a large volume before operation can begin. For this type of reservoir only a small portion of the water held behind the dam is useful. Therefore, knowing the purpose for which a reservoir has been constructed, and knowing how much water can be used for that purpose, helps determine how much water is in possible reserve.
The following terms are used in connection with the volume of reservoirs:
The list below largely ignores many natural lakes that have been augmented with the addition of a relatively minor dam. For example, a small dam, two hydroelectric plants, and locks on the outlet of Lake Superior make it possible to artificially control the lake level. Certainly, the great majority of the lake is natural. However, the control of water that can be held in reserve means a portion of the vast lake functions as a reservoir.
Recognition of lakes like Lake Superior greatly changes the list below. For example, the Francis H. Clergue Generating Station and Saint Marys Falls Hydropower Plant, which are both on the lake's outlet, operate with just 5.9 meters total head. This is short compared to other dams. However, when viewed against the 81,200 km2 area of the lake, even a small range in Lake Superior's water level means its active volume is greater than the largest nominal in the table below.