However, nothing of this sort is observed, for example in the case of the Amu Dar'ya. During periods when its erosion level dropped, regardless of whether it flowed into the Aral Sea, Lake Sarykamysh, or the Caspian, this river (or more precisely its corresponding branches) filled up its channel with deposits rather than deepening it, the accumulation of deposits being the most intensive in the head reaches of the various branches. Thus it is clear that in this case the accumulation of deposits was in no way related to variations in the erosion level, and that it was determined solely by a decrease in the stream velocity (that is, by a reduction of the total amount of water flowing into the given branch). In other words, the channel processes do not cause a reduction of the water discharge, but rather the opposite is true: a reduction of the discharge produces the corresponding channel processes.
A study of the ancient irrigation, and also certain historical data, indicate that the extensive irrigation constructions in the delta regions of the Amu Dar'ya, the Syr Dar'ya, and other rivers were erected at times when these localities had received an abundant natural supply of water for a long time. In those times the problem was not so much to ensure a sufficient supply of water as to ensure its maximum utilization. However, as time went by, this well-watered phase of the cycle was succeeded by another phase in the utilization of the given region for irrigation farming: a poorly watered phase during which the total influx of water to the canals gradually became less. Then the problem was one of ensuring a sufficient supply of water. In order to solve this problem the inhabitants were compelled to carry out large-scale clearing and deepening of the canals, to move the canal heads upstream, to erect dams, etc.
As pointed out by Tolstov (1947), the inhabitants then apparently hauled up and brought back to their fields the gradually disappearing water of the drying branches of the river. Thus, in spite of the fact that during the second phase man, by means of his intervention, even improved the water supply conditions in this area, this supply nevertheless diminished, and after a time it was cut off completely, for reasons which had nothing to do with human activity. Man in this case only succeeded in retarding, to some extent, the naturally occurring process.
Makeev (1952, p. 554-562) thinks that the abandonment of lands for socioeconomic reasons cannot create natural conditions which will prevent their subsequent irrigation. When lands suitable for irrigation are abandoned for centuries, this just indicates that it is impossible to supply these lands with water without carrying out large-scale operations. The people who live by working this land will not be easily induced to leave the cultivated area, and they will take all possible measures to continue its irrigation. The previously cultivated lands on the Kunya Dar'ya and the Zhana Dar'ya apparently became neglected because the flow of water along these channels ceased, as a result of natural causes.
We have seen that certain regularities in the development of all rivers, for instance the simultaneity of their changes in course and the fact that these changes are confined to definite climatic periods (equatorward deviation of rivers during warm periods and poleward deviation during cold periods), can be accounted for neither by a slackening of the regulative activity of man nor by any kind of channel processes. Thus it follows that climatically induced fluctuations in the length of the terrestrial day must be the main, or background, cause of periodic river migrations on a world-wide scale.
We have pointed out that the course of the Earth's evolution, with respect to many of its important features, is determined by three regulari ties. This has been the case ever since a certain stage in the evolution of our planet when water appeared on the Earth's surface and the division of this surface into dry land and sea took place. From this time on, it became possible for water to change from liquid to solid (snow, firn, or ice), and vice versa. Thus conditions were created which were conducive to periodic redistributions of the water mass relative to the poles and the equator (via hydroatmospheric processes produced by a change in the overall external thermal balance of the Earth). These important regularities in the evolution of the Earth may now be summarized as follows: 1. First-order regularity in the evolution of the Earth. Three factors, climate, diurnal-rotation regime, and tectonics, stand in a mutual cause effect relationship to one another. Any variation in one of these factors, no matter what its cause, will inevitably entail corresponding variations in each of the others.
The authors consider it their pleasant duty to express their warmest and most heartfelt thanks to B. A. Apollov, B. L. Lichkov, N. I. Makkaveev, K. K. Markov, and A. V. Shnitnikov for reading through the manuscript of this article and for giving valuable advice and recommendations.