Anciently its course must have been still longer.

The Oxus, in the time of the Achaemenian kings, fell into the Caspian by a channel which can even now be traced.

Its length was thus increased by at least 450 miles, and, exceeding that of the Jaxartes, fell but little short of the length of the Indus.
The Oxus, like the Nile and the Indus, has a periodical swell, which lasts from May to October.

It does not, however, overflow its hanks.

Under a scientific system of irrigation it is probable that a considerable belt of land on either side of its course might be brought under cultivation.