Geological History and Present Conditions of Lake Balkhash

Abstract

Lake Balkhash is a large endorheic water body, the third largest by size in Eurasia and the second largest salt lake of the world. With its half-moon elongated morphology and 78% of inflows provided by the Ili River from the West, the lake has a freshwater basin in the West and saline water basin in the East, separated by the 4-km narrow and 6-m deep Uzunaral Strait. The average bathymetry is shallow, with a maximum water depth of 11 m in the West and 26 m in the East. According to investigations of the geological history of the lake in Soviet times and in international projects during the last 15 years, a large lake was formed by the Ili River in the Balkhash region encompassing the present area of the Kapchagai Reservoir during the Middle Pleistocene. The large lake basin was subsequently transformed by a series of tectonic deformations. Around 300 kiloyears before present (ka BP) the Ili River was diverted to the North where it formed a large megalake, the Ancient Balkhash, in the Balkhash-Alakol Depression. Around 110 ka BP, the lake became divided into two basins forming the Alakol Lake in the East and the modern Lake Balkhash in the West. Hydrological conditions mostly controlled by precipitation, evaporation and meltwater discharge caused three different lake-level stages in the Late Pleistocene and Holocene: lake levels between 349 and 355 m above sea level (asl) prevailed during glacial periods, between 341 and 348 m asl during the pluvial early and middle Holocene, and between 335 and 348 m asl during the arid late Holocene when extreme regressions at ca. 5.0, 1.2 and 0.8 ka BP divided the lake into more than one basin. The present lake water balance results from a major regression due to a recent phase of aridization and the filling of the Kapchagai Reservoir in the 1970s and 1980s and the compensation of lower precipitation by increased meltwater discharge from glaciers. However, meltwater runoff will diminish with rapidly shrinking glaciers in the next 50 years. This alarming perspective requires careful water-basin management which was not yet implemented. Lake Balkhash is exposed to the threat of exaggerated anthropogenic water subtraction due to an accelerated infrastructural and demographic boost that doubled the irrigated farmlands in the Chinese part of the catchment in less than 20 years. Due to the lake’s hydrology, catchment rock and hydrochemical conditions, the water of the Eastern Balkhash has high concentrations of potassium and magnesium, unfavorable for hydrobionts. Any further increase in salinity will soon cause a considerable diminution of the lake’s biomass. The ichthyofauna of the lake has been intensively manipulated during the twentieth century, with the introduction of new species and the decline of the original ones. The substitution of the native fish fauna by introduced species caused a decrease of valuable commercial fish in the lake and a decrease of the total fish catch. Thus, Lake Balkhash faces serious environmental risks today and its near future depends on the collective will and decisions of the responsible agencies.