There are a range of factors that influence the feasibility and economics of a proposed reservoir site. The most important of these is generally the location of the dam. After that, consideration must be given to the run-off characteristics of the catchment area, the watertightness of the proposed reservoir basin, the stability of the valley sides, the likely rate of sedimentation in the new reservoir, the quality of the water and, if it is to be a very large reservoir, the possibility of associated seismic activity. Once these factors have been assessed, they must be weighed against the present land use and social factors. The purposes that the reservoir will serve must also be taken into account in such a survey.

Although most reservoirs today serve multiple purposes, their principal function, no matter what their size, is to stabilize the flow of water, firstly, to satisfy a varying demand from consumers and, secondly, to regulate water supplied to a river course. In other words, water is stored at times of excess flow to conserve it for later release at times of low flow, or to reduce flood damage downstream.

The most important physical characteristic of a reservoir is its storage capacity. Probably the most important aspect of storage in reservoir design is the relationship between capacity and yield. The yield is the quantity of water that a reservoir can supply at any given time. The maximum possible yield equals the mean inflow less evaporation and seepage loss. In any...