During the postglacial period, and in particular during the last 4500 years, the Amu Dar'ya also deviated southward whenever there was a time of climatic cooling (at the end of the Neolithic, in antiquity, and in the late Middle Ages). Thus the region around the Sarykamysh delta became watered and to some extent the Sarykamysh basin itself; the water level in the latter sometimes became so high that the excess overflowed into the Uzboi and on into the Caspian. During each of the three warm periods (Bronze Age, early Middle Ages, and the present), on the other hand, the river deviated northward. As a consequence, its left bank (the Sarykamysh delta) dried up and its right bank received more water.
The foregoing conclusions are borne out by historical-archaeological and geomorphological studies carried out for the region in question; such studies have been made by Gulyamov, Tolstov, Kes', Zhdanko, Itina, Trombachev, Lents, and others.
Along the Uzboi Bronze-Age remains are found considerably more rarely than Neolithic remains, whereas on the Akcha Dar'ya delta there were more Bronze-Age settlements. This indicates that during the Bronze Age there was much less water in the Uzboi, and that most of the water flowed out to the north, to the Aral Sea, or else along the Akcha Dar'ya distributaries. In ancient, times, however, the Akcha Dar'ya ceased to exist, as indicated by the canals built at this time, which led directly from the Amu Dar'ya (in its present course). All the main left-bank distributaries of the Sarykamysh delta, on the other hand, were active at this time. Most of the water of the Amu Dar'ya flowed via the old Daudan stream into the Sarykamysh basin, and then out along the Uzboi (some of the water may even have bypassed the basin).
The abundant supply of water which resulted from the southward deviation of the Amu Dar'ya enabled the ancient inhabitants of Khorezm to construct a magnificent irrigation system within the vast environs of the Sarykamysh delta. In this region irrigation farming ensured a flourishing economy at the time. In the early Middle Ages, however, with the onset of the next warm period, the river deviated to the right and the region around the Sarykamysh delta was no longer supplied with water.
The inhabitants of Khorezm then made strenuous efforts to prolong the operation of the irrigation system (by deepening the canals, by transferring the main ducts upstream, by introducing hoisting wheels to raise the water, etc.). However, nevertheless, most of the irrigated regions finally had
to be abandoned, and the settlers moved either to distant places where there was water or else to the right bank of the Amu Dar'ya, where condi tions were better at that time. A new irrigation system was then constructed, which included, among others, the large Kurder canal. During this period the Sarykamysh channel of the Amu Dar'ya, Lake Sarykamysh, and the Uzboi River did not exist. The land in the Sarykamysh delta became well watered again only with the onset of the next cold period (late Middle Ages). "Following the catastrophic curtail ment of irrigation in the fourth to eighth centuries, the irrigation system in the territory of the Sarykamysh delta was reconstructed and expanded again, but it did not reach anywhere near the scale of the ancient system" (Tolstov, Kes', et al., 1960).
During the present warm period the Amu Dar'ya once more has a tendency to deviate to the right. In the middle of the last century this led to, first, a catastrophic flooding of the rich farmlands in the Kushkan Tau hills, which were inhabited by Karakalpaks, and, second, to a complete lack of water in the Khan Abad region, situated more to the west. In an attempt to retain the disappearing water, the inhabitants of northwestern Khorezm constructed dams, but to no avail. The Kungrad and Shomonai farming regions were destined to be abandoned.
As the Amu Dar'ya shifts to the right, it appears to ascend an inclined plane. The great height of the right bank is indicative of the fact that this bank is being undercut by the river. During the last hundred years the Amu Dar'ya has moved about 10 km to the east in the Khozaraps-Urgench region. The rightward deviation of the river is in evidence all along the lower reaches of the Amu Dar'ya; this has made it necessary to extend the existing inlet channels to the irrigation ditches or to dig new ones. The periodic migrations of the river have inevitably entailed a resettling of the local population, whose lives and activities depended to some extent or other upon the water of the Amu Dar'ya.
The Aral Sea. This body of water is located in the southeastern part of the Aral-Caspian depressiot_. Its water supply depends on the surface flow and subsurface flow into the area.