It is believed that the Amu Darya's course across the Kara-Kum Desert has gone through several major shifts in the past few thousand years. Much of the time – most recently from the 13th century to the late 16th century – the Amu Darya emptied into both the Aral and the Caspian Seas, reaching the latter via a large distributary called the Uzboy River. The Uzboy splits off from the main channel just south of the river's delta. Sometimes the flow through the two branches was more or less equal, but often most of the Amu Darya's flow split to the west and flowed into the Caspian.