Ordinarily when water is exposed at the surface in highly arid environments it becomes saline due to a high rate of evaporation. In this case, even though the rate of evaporation from Lake Yoa is equivalent to 6 metres (20 ft) a year, the total lake depth is 25 metres (82 ft), unique physical factors combine to keep all of the lakes, except the central Lake Teli, fresh. First, water accumulated in an underground aquifer during the wet millennia is supplied to the lakes.