There are two conditions deemed as necessary for the formation of lakes – (i) Presence of a basin to hold water and there must be a supply of water to fill or partially fill the basin, (ii) The bottom of the basin should be below the outlet so long as the basin exists.
Most of the salt lakes are characterized by the absence of outlets. In salt lakes evaporation is so strong that the lake is never allowed to be filled with water.
In semi-arid areas bordering the hot deserts, lakes are generally seasonal in nature. During the rainy period, water may accumulate in a natural basin, but during the hot and dry season the lake may shrink in volume or may disappear.
For example, Lake Chad because of its location on the border of Nigeria and French Equatorial Africa shrinks in volume from 50000 sq. miles in rainy-season to 10000 sq. miles in dry season.
Remember that the lakes of inland drainage areas which do not have river outlet to sea are extremely saline as a result of high rate of evaporation and low intake of fresh water. In such lakes various salts brought into them by rivers remain there, while water is lost by evaporation.
The permanence of lakes depends on two important factors i.e. their storage capacity or the size of the basin, and on the relationship between the amount of water received (either from rain or from their inflowing streams) and the amount lost directly by evaporation or by their outlets. Seepage may also account for the loss of water from a lake.