The Aydar Lake (Uzbek: Aydar Ko‘l; alternate spellings: Lake Aydarkul, Lake Aidarkul) is part of the man-made Aydar-Arnasay system of lakes, which covers an area of 4,000 square kilometres (1,500 mi²). This system includes 3 brackish water lakes (Aydar Kul, Arnasay and Tuzkan) located in the saline depressions of the south-eastern Kyzyl Kum (now in Uzbekistan and Kazakhstan). The lakes are the unintentional byproduct of Soviet planning.
Up to the middle of the last century the Arnasay lowland remained a dry Salt pan during most of the year. Only in Spring, in the lowlands, would the small, ephemeral Lake Tuzkan glisten briefly, disappearing in the hot weather.
In the early sixties the Syr Darya was dammed up. Simultaneously the Chardarya irrigation dam was constructed. Floodgates were provided in the dam for flood control, and when in 1969 a raging flood occurred, these were opened as the dam's capacity was inadequate to cope with the flow. Between February 1969 and February 1970 almost 60% of the Syr Darya's average annual water flow (21 km³) was drained from the Chardarya Reservoir into the Arnasay lowland. In such a way new lakes were unintentionally created. Since 1969 the Aydar Lake has regularly received the waters of the Syr Darya River when they overflow the capacity of the Chardarya Reservoir. This has gradually filled up the natural cavity of Arnasay lowland to create the second largest lake in the region (after the remains of the Aral Sea).
In 2005 the Aydar Lake contained 44.3 cubic kilometers of water. Today the area of the Aydar Lake amounts 3,000 square kilometres (1,200 mi²). It is nearly 250 km (160 mi) long and up to 15 km (9.3 mi) wide. The mineralization of the water in the Aydar Lake averages only 2 grams per liter (2,000 ppm).