In arid regions the lakes have no surface outlets. In such regions the average rate of water loss by evaporation balances the average rate of stream inflow.
However, if the rate of inflow increases, the lake level will rise with the result that the lake surface will increase in area allowing an increased rate of evaporation. A new balance can then be achieved. If the aridity in the region increases, the input is reduced and evaporation is increased, so the water level will fall to a lower level.
Dissolved solids are brought into the lake by streams-usually streams with their heads in distant highlands where there is water surplus. Since only pure water is removed by evaporation, the salts remain behind and salinity of the water slowly increases. Eventually, salinity level reaches a point where salts are precipitated as solids.
The Dead Sea with a surface elevation of – 396 m is saline. Similarly, the largest of all lakes, the Caspian Sea has a surface elevation of – 25 m, and is saline.
In such regions in which climatic conditions are such that evaporation always exceeds precipitation, the lakes may be absent. Instead of lakes, a shallow basin covered with salt deposits, known as dry lake or salt flat will occur.
It is only on rare occasions, these flats are covered by a shallow layer of water, brought by flooding streams that originate from adjacent highlands.
The Aral Sea is an inland lake that is becoming saline through human activity. The two major rivers that flow into this lake have been largely diverted into irrigation of agricultural lands.
This has resulted in reduction of their inflow, so the lake has shrunk and become increasingly salty.