In spite of the increased efforts of its users to adapt the irrigation system to the new hydrological conditions, due to the lack of water the system was no longer able to perform its function, namely to irrigate the fields. Thus farming declined and the whole region became desolate. Numerous remains of ancient towns and settlements, as well as of irrigation structures, all dating from antiquity and from the late Middle Ages, attest to what took place in this region. The fact that there are no such remains from the warm period of the early Middle Ages on the Kuvan Dar'ya and Zhana Dar'ya deltas, moreover, is further evidence of this.
The tendency of the Syr Dar'ya to migrate during the present warm period is verified by the direct observations of various investigators (for example, Bogdanov (1882)). The northward deviation of the Amu Darya during this period was accompanied by a deviation of the Syr Dar'ya in the same direction. The Sarysu and the Chu gradually became separated from the Syr Dar'ya, and the Yany Darya and Zhana Darya became overgrown. The Syr Dar'ya found a new course along the Kuvan Darya, but then it abandoned this course as well and found its present channel. As a consequence, the Yany Dar'ya and Kuvan Dar'ya dried up. It should be recalled that Myshkin's map for the year 1831 showed the Syr Dar'ya discharging into the Aral Sea via three branches (the Zhana Dar'ya, the Kuvan Darya, and the Syr Dar'ya).
Recently, the channels on the right side of the Syr Darya have developed considerably. For example, the Kara Uzyak, which was formed at the site of the extensive floods at the end of the last century, carried only 10% of the discharge of the Syr Dar'ya in 1925, whereas at present it carries 40%. This development of the channel, moreover, is continuing.
Borovskii and Pogrebenskii (1958) think that the bulk of the water of the river is in the process of shifting over from the Syr Darya channel to the new, growing Kara Uzyak channel. In connection with this shift to the Kara Uzyak, it may be assumed that the Dzhaman Dar'ya, the main channel of the Syr Dar'ya, will become silted up at its beginning.
The Kara Uzyak and Kok Su distributaries will undergo an intensive development, deepening and widening their channels and thereby reinforcing the discharge to the right. This shifting of the distributaries northward may also continue further, as far as the Kara Kemir bench.
The Amu Dar'ya. In its lower reaches this river flows northwest and discharges into the Aral Sea (Figure 8). It is a known fact that the Amu Dar'ya has repeatedly and abruptly changed course and migrated over a vast territory in the western plains of Central Asia, and that these plains were formed primarily by the alluvial deposits of this great river.
On the basis of the correlation between the cyclical fluctuations of the climate and the planet-wide rearrangements of the drainage system, it is possible to draw up the following approximate plan of the successive shifts in the course of the Amu Darya during the Quaternary period.* The period of Dnieper glaciation was characterized by an enormous accumulation of solid moisture in the high latitudes. Accordingly, the Earth's diurnal velocity was relatively high, and the global drainage system shifted equatorward under the influence of the centrifugal forces. The Amu Dar'ya also shifted in this direction at that time. Pressing on to the foot of the Kopet Dagh, it flowed into the southwestern depressions of the Aral-Caspian plain and filled them up with alluvial deposits (see Figure 8a).
The period of Dnieper glaciation was succeeded by a warm interglacial period. The solid moisture in latitudes then turned into liquid and part of it was transported to the low latitudes. As a result, the diurnal rotation of the Earth slowed down and the centrifugal forces were reduced accordingly. The drainage system responded to the new rotational regime of the Earth by shifting toward the poles. During this epoch the Amu Dar'ya deviated to the right (northward), and the depressions located in the northwestern part of the Aral-Caspian plain began to receive its discharge. At that time it flowed into these depressions and filled them up with alluvial deposits (Figure 8d).
The warm interglacial period gave way in turn to a new wave of climatic cooling. However, the glaciation at this time was on a smaller scale than during the preceding glacial period. Consequently, the equatorward shift of the global drainage system was also less.
The Amu Dar'ya then deviated to the left and flowed first into Lake Sarykamysh and afterwards along the Uzboi to the Caspian Sea (Figures 8b, 8c); at this time the central and southwestern depressions of the Aral Caspian plain were watered and filled in with sediments. Thus, the river then occupied an intermediate position between the following two extremes: a north-meridional course (corresponding to the warm interglacial period) which lay approximately along the Akcha Dar'ya channel, and a latitudinal course (corresponding to the period of the Dnieper maximum of climatic cooling) along the foothills of the Kopet Dagh.