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, the most recent period being in the 13th century to the late 16th century, the amu darya emptied into both the aral and the caspian seas. 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.