The lakes are probably groundwater-fed, as evaporation in the area greatly exceeds precipitation, with the lakes losing about water per year. Freshwater springs nourish the lakes. At least one water body was reported to be fresh in 1951 while the others are warm and saline. Deuterium isotope ratio analysis indicates that the water at Waw an Namus is recent water, certainly more recent than 8,000 years.