Caspian Sea
The Caspian has characteristics common to both seas and lakes. It is often listed as the world's largest lake, although it is not a freshwater lake. It contains about 3.5 times more water, by volume, than all five of North America's Great Lakes combined. The Caspian was once part of the Tethys Ocean, but became landlocked about 5.5 million years ago due to plate tectonics. The Volga River (about 80% of the inflow) and the Ural River discharge into the Caspian Sea, but it has no natural outflow other than by evaporation. Thus the Caspian ecosystem is a closed basin, with its own sea level history that is independent of the eustatic level of the world's oceans.
Uzboy
At various times in the past one or more branches of the Oxus split off from the Oxus delta and flowed west into the fresh-water Sarykamysh Lake. This was drained south by the Uzboy River about 175 km to near Igdi where it turned and flowed about 290 km west, reaching the Caspian at Krasnovodsk Bay through the Bala-Ishem salt marshes. There was a large waterfall, a place called the Lion’s Jaw and at one point the river disappeared underground for some distance. (Glukhovskoy does not mention it, but GoogleEarth shows a dry river bed that starts about 100 miles west of the Oxus and runs about 400 miles west to join the Uzboy near Igdi.) All geographers, from the ancient Greeks to the early Arabs, reported that the Oxus flowed into the Caspian, although their accounts are vague. From the 10th to the 13th centuries there are no reports of a Caspian mouth. It is thought that a dam was built near the old capital of Konya-Urgench and that this dam was destroyed by the Mongols when they sacked the city in 1220. We again hear of a Caspian mouth from about 1310 to 1575. It is possible that the main current shifted from the Aral to the Caspian, causing the Caspian to rise and the Aral to sink. There may have been a significant population along the Uzboy and farmland in what were later the marshes at the north end of the Oxus delta. It seems that about 1575 the east branch of the Oxus cut through some hills causing the main current to shift to the Aral Sea. Also a dam was built near the old site to retain the remaining water and to deny it to the Turkomans who were in the habit of raiding the Oxus delta. This not only blocked the Uzboy but caused the Sarikamish to slowly dry up.