1. Natural lakes.
2. Natural depressions on main drainage lines.
3. Natural depressions on lateral drainage lines.
4. Arbitrary and artificially constructed basins.
Natural lakes may need to be investigated more or less thoroughly to determine the character of their waters, whether saline, alkaline or fresh. It may also be necessary to know their normal depth and capacity, and to make a study of their outlet if they have one. In some instances the storage capacity of a lake may be enormously increased by means of a comparatively low and inexpensive embankment.
The area of reservoir basin, mean depth, temperature of the water, exposure of wind and sunshine, losses by seepage and evaporation, all have a bearing upon the available water supply and influence the design of the dam and accessories to the reservoir.
In determining the character and suitability of materials for constructing a dam it is necessary to make a careful study of the soil and geological formation. This is best accomplished by digging numerous test pits over the basin, especially in the vicinity of the proposed dam site; borings alone should never be relied upon for this information. For such an investigation the advisability of borrowing material for dam construction from the reservoir basin is determined. The porous character of the subsoil strata, or the dip and nature of the bed rock, may forbid the removal of material from the floor of the basin, even at a remote distance from the dam site.
The area to be flooded should be cleared and grubbed more or less thoroughly, depending again upon the use for which the water is impounded. In no instance should timber be left standing below the high-water level of the reservoir; and all rubbish liable to float and obstruct the outlet tunnel and spillway during a time of flood should be removed.
The accessories to a reservoir, to which reference has been made, may be enumerated as follows:
1. Outlet pipes or tunnel.