An accidental inland sea
In the arid expanse of Uzbekistan's Kyzylkum Desert lies a body of water nearly 250 kilometers long: Lake Aydarkul. The lake is not natural. Before 1969, the Aydar depression was a dry salt pan, where a small, ephemeral lake named Tuzkan that appeared in spring and vanished by summer. The lake's existence resulted from a Soviet-era engineering and agricultural project that went unexpectedly wrong.
During the 1960s, the USSR constructed the Chardara Reservoir on the Syr Darya river to support massive cotton cultivation projects. In the winter and spring of 1969, unusually heavy snowmelt from the Tian Shan mountains caused catastrophic flooding. The reservoir's capacity was overwhelmed. To prevent the dam from breaking and inundating downstream communities, engineers made the emergency decision to open the floodgates and divert the excess water into the vast, low-lying Arnasay depression.
Between February 1969 and March 1970, about 21 cubic kilometers of water—nearly 60% of the Syr Darya's average annual flow—poured into the salt pan. This single event transformed the desert landscape, creating the Aydar-Arnasay Lake System, with Aydarkul as its largest component. The new lake system reached a volume of 44.3 cubic kilometers by 2005 and now covers an area of around 3,000 square kilometers.
A chemically divided lake
Lake Aydarkul's unique hydrology creates a phenomenon known as chemical stratification. The water column is separated into distinct, invisible layers that do not mix, a condition called meromixis. This layering is not caused by temperature differences, but by a sharp gradient in salinity, forming what is known as a halocline.
The stratification comes from how the lake receives and loses water. Inflows from the Syr Darya and agricultural runoff are relatively fresh, while the original basin was a salt pan. With no natural outlet, the lake's only water loss is through evaporation, which leaves dissolved salts behind. This process concentrates salts over time.
The less dense, fresher water stays at the surface, while the saltier, denser water sinks to the bottom. This density difference is strong enough to resist mixing from wind and seasonal temperature changes. As a result, there is a measurable salinity gradient across the lake. In the eastern parts, closer to the freshwater inflows, mineralization is around 3 to 5 grams per liter. In the more isolated western parts of the lake, salinity can exceed 8 grams per liter. This stable stratification means the deeper layers are cut off from the atmosphere, often leading to anoxic (oxygen-deprived) conditions that can only support specialized microorganisms.