Changes in the form and biota of the Aral Sea over time

Changes in the biota of the Aral Sea can only be understood from knowledge of the palaeolimnology of this lake. The evolution of a terminal lake in arid and semi-arid areas depends upon the evolution of river basins and the catchment areas of those rivers discharging into the lake. Generally in arid areas, the combined input from rainfall, surface and deeper groundwaters is considerably less than the input from rivers. Examples are provided by Lake Chad with two rivers, and the Dead Sea with one. In the case of the Aral Sea, two major factors have determined the history of the Amu and Syr Dar’ya basins: local tectonic movements, and alternation between pluvial and arid climatic phases (the storage of water in ice-fields at high altitudes within the catchment areas was also important during glacial times).