In general, during the course of geological time the hydrosphere influences the diurnal-rotation regime in two ways. First of all, it transforms gravitational energy into mechanical energy of the diurnal rotation: it slows down, or brakes, this rotation. Second, the hydrosphere becomes shifted poleward or equatorward to some extent or other, as a result of hydroatmospheric processes, thereby changing the moment of inertia of the rotating body (the Earth). Thus, ultimately it transforms solar thermal energy into mechanical energy of diurnal rotation, and it imparts a fluctuational character to this rotation. Consequently, the observational data indicating that the secular variations in the Earth's rotational regime cannot be accounted for solely in terms of a tidal mechanism are quite to be expected.
At present it is difficult to judge to what extent the so-called spasmodic variations in the angular velocity of the Earth, which can become appreciable over the course of some years, are climatically induced. The correlation of these variations with climatic fluctuations during the last two or three centuries (and also their correlation with the level fluctuations of the ocean and the Caspian Sea), however, has been demonstrated convincingly by Maksimov (1954, 1960). It is interesting that the drainage system even responds appreciably to these fluctuations in the Earth's diurnal-rotation regime. For example, when the Earth's rotation speeded up considerably during the 1870's and 1880's (during the five years from 1870 to 1875, the Earth "gained" five seconds), the drainage system "responded" to this with a definite tendency toward an equatorward shift.
In addition to the overall, or background, factor (the variations in the centrifugal forces of diurnal rotation), a great many other factors affect the evolution of the drainage system. These factors influence, in particular, migrations of rivers (especially in their lower reaches) and level variations in water bodies. In the past the influence of the rotational regime of the Earth on the evolution of the planet was not taken into account. Thus, insufficiently substantiated hypotheses were advanced to explain the hydrological phenomena in specific cases, or (what was more frequent) the roles of certain factors were exaggerated.
For example, some archaeologists and geomorphologists (Tolstov, Kes', and others) think that the repeated variations in the courses of the Amu Dar'ya and Syr Dar'ya are principally due to two factors: the accumulation of alluvial deposits, and human intervention. For historical times certain
investigators suggest that the second factor (that is, sociopolitical causes) is the most important one. Tolstov (1960) is inclined to explain the migrations of the rivers by the way in which the deltas are formed, since an extremely rapid accumulation of deposits takes place in the deltas, especially in the immediate vicinity of the channels and in the channels themselves. As a result of this the stream very rapidly reaches the top of the embankment around the channel, and of course it attempts to continue its flow via lower places. According to Tolstov, during historical times, when man used the Amu Dar'ya to irrigate his fields, the channel was no longer able to migrate. Man regulated the flow of the river, in an attempt to utilize it in the required direction. Consequently, each time the Amu Dar'ya briefly broke through and flowed along its old channels during historical times, this was related to a period of sudden slackening of the regulative activity of man, as a result of major sociopolitical catastrophes.
Now let us consider in somewhat more detail what parts these two factors actually play in the evolution of the global drainage system. First let us consider channel processes. During the course of its evolution a river tends to develop a so-called equilibrium profile. The river usually enters into this more or less stable state first in its lower reaches and then gradually in its upper reaches as well. If the influx-discharge condi tions (that is, the wetness conditions in the basin of the river in question) do not vary as time goes by, and if there are no tectonic movements, the equilibrium profile will depend entirely on the erosion level of the river.
If the erosion level drops, downcutting of the river bed will gradually take place, beginning in the lower reaches, and the elevation of the channel will drop accordingly. If the erosion level of the river rises, on the other hand, then (also beginning in the lower reaches) the channel will gradually begin to fill up with deposits, and its elevation will rise. Thus, the accumulation of sediments in a channel begins only when the water level rises inthe body of standing water, as a result of filling of the latter with solid deposits, the erosion level of the channel being raised accordingly. Only in such a case will the elevation of the channel be raised and conditions will be favorable for shifting of the stream to a new channel, the probability of a shift in one direction being the same as that of a shift in the other.