How did changes in the courses of the Amu Darya and Syr Darya affect the Aral Sea long before modern irrigation?

Long before modern, large-scale irrigation projects began in the 20th century, the water levels of the Aral Sea were primarily influenced by natural fluctuations in the inflow of the Amu Darya and Syr Darya rivers.

Historically, the most significant natural change occurred between the end of the Pleistocene Epoch and the early Holocene Epoch (after about 11,700 years ago). During this period, the Amu Darya temporarily shifted its course from the Caspian Sea to the Aral Sea, which resulted in the lake being inundated with water for the first time.

Following this event, the combined flows of the two rivers generally maintained a high water level in the sea for millennia. While the region experienced relatively brief dry spells between the 3rd and 1st centuries BCE, the lake’s water balance remained stable until the mid-20th century, as the quantity of water brought in by the rivers was roughly equal to the amount lost through evaporation.