If we ignore the case of lakes for recreational purposes and small dams for hydroelectric power generation, reservoir storage is the main factor influencing the entire dam design. The objective is in fact to have a volume of water  available for increasing dry weather river flow, irrigation or drinking-water supply, or free storage capacity to attenuate flooding.

The first task therefore consists in calculating the volume of water that can be stored in a basin, possibly at several different sites. A first approximation can be achieved using a 1/25 000 scale map with contour lines every 5 or 10 metres, except for reservoirs with storage of several tens of thousand cubic metres. The second task will then be to check whether conditions in the catchment area are such that the reservoir will be filled and to calculate the risk of shortfall.

Except for severely jointed rock or rock with very mediocre characteristics, rock foundations are suitable for construction of any type of dam, provided that suitable measures are taken to strip off severely weathered materials and, if necessary, treat the foundation by grouting. Fill dams will always be suitable. For the other types, requirements are more severe for RCC, still more for conventional concrete, and finally most stringent for arch dams. The most important aspect is cracking (faults, joints, schistosity).

Availability, on the site or near it, of suitable materials to build the dam has a considerable influence and one that is often decisive in choosing the type of dam:

If it is possible to extract the materials from the reservoir itself, reservoir storage can be increased. This also usually keeps the cost of transport and restoring borrow areas to a minimum.

As a general rule, if silty or clay soil of satisfactory quality (fines content, plasticity, condition) and quantity (1.5 times or twice the volume of fill required) is available, a dam construction alternative using homogeneous earthfill or quarry-sorted materials – setting aside the coarsest materials for the downstream shoulder – will be the most economical provided that the flood flows to be discharged are moderate.

If only a limited quantity of impermeable materials, and coarse or rockfill materials as well, is available, the possibility of a zoned earthfill dam or a rockfill dam with a watertight core can be considered. The disadvantage of this alternative is placement in zones, which is all the more complicated when the site is narrow, hindering movement of the machinery.

If only coarse materials are available, they can be used to build a homogeneous embankment with a watertight diaphragm wall built in the centre of the dam, by grouting after the fill has been placed or by an upstream watertight structure (concrete or bituminous concrete facing).

If only rockfill is available, a compacted rockfill dam with external watertight structure (geomembrane, hydraulic concrete or bituminous concrete facing) on the upstream face, will be suitable. A concrete alternative, especially RCC, can also prove to be competitive provided the foundation is good enough (rock or compact ground) with no need for excessive excavation.