The effect which fluctuations in the rotational regime have on the shifting of the coastline must be universal and must extend over the entire surface of the spheroid. Other effects, on the contrary, are usually localized within definite regions or else are minor. In addition, the division of the Earth's surface into regions of regression and transgression of the oceans must manifest itself as a preponderance of either inward or outward shifts of the coast in different latitudes of the spheroid. In spite of this, so far it has not been possible to draw any important conclusions concerning the rotational regime of the Earth just on the basis of coastline shifts; there are too many masking factors which influence these shifts.
There is one hydrospheric phenomenon, however, which clearly and unambiguously indicates the existence of long-period variations in the length of the day; this is the planet-wide rearrangement of the drainage system.
The rotational forces which affect the entire hydrosphere as a whole (and shift it relative to the lithosphere) will naturally have a corresponding effect on parts of the hydrosphere considered separately (for instance, on the ground water and surface water of the continents). Thus, the whole hydrographic system of the Earth must periodically undergo a more or less thorough rearrangement. Such a rearrangement will naturally take place under the influence of other factors as well, for example, as a result of tectonic movements, fluctuations of the base level, filling in of individual parts of channels and basins with river sediment, human intervention, etc. However, there is an important fundamental difference between the effects of all these other factors and the effect of the rotational forces. The rearrangement of the drainage system caused by variations in the rotational regime of the Earth takes place regularly and on a world-wide scale, its intensity being the greatest in the middle latitudes. The effects of the various other factors, on the other hand, are neither regular nor universal.
When the Earth's rotation on its axis slows down, the whole hydrosphere (oceans and seas, lakes and rivers, ground and surface water) has a tendency to shift toward the poles, due to the decrease in the centrifugal forces. The water divides of all basins are shifted in the direction of the equator, and the poleward liquid and solid runoff increases. For all rivers the erosion of the northern banks will increase in the Northern Hemisphere and that of the southern banks will increase in the Southern Hemisphere. In individual parts of rivers, especially in their lower
reaches, there is an increased tendency toward breaking through the banks, leading to changes in the direction of the flow, and consequently to shifts (generally poleward) in the locations of the deltas and mouths. As a result, the so-called "wandering" rivers and lakes are formed.
For rivers flowing toward the equator, the following takes place: the stream velocity is reduced; the accumulation of sediments and the filling in of river valleys with them increases; the bed becomes more sinuous and unsteady; the mouth becomes more branched, with shallows; and the rate of advance of the delta into the standing-water body decreases. Consequently, rivers of this type will appear to be senile.
Rivers flowing poleward, however, will cut their way into the bed more strongly, making it deeper and cutting out stream terraces; the stream velocity will increase; the bed will become more rectilinear and more stable; and the rate of advance of the delta will increase. Rivers of this type will in general appear to be in a state of rejuvenation. When the diurnal rotation of the Earth speeds up, on the other hand, the centrifugal forces become greater and all the aging phenomena described previously will be observed for these rivers.
Let us now illustrate how a change in the direction of a river takes . place, and how river mouths and deltas become modified. We shall also indicate how in some cases bodies of standing water disappear and are replaced by new ones. Consider a river flowing in a latitudinal direction. In the plain portion of its lower reaches the river divides into two branches, one northern and one southern. When the diurnal-rotation velocity of the Earth increases, the "base supply" of the river (the ground water of the alluvial valley-depression through which the river flows) becomes shifted toward the equator. Consequently, the water table in the part of the valley through which the northern branch flows is lowered. The discharge of water and the stream velocity are reduced, as a result of the increase in infiltration losses; thus there is a progressive increase in sediment accumulation along the river. Drainage to a body of standing water becomes more difficult, and the branch enters into a stage of natural drying up. This cannot but have a direct effect on the regime (and on the general fate) of the body of water into which the river discharges, especially if it is landlocked.