Some Physicogeographic Facts

The changes undergone by the southward-flowing rivers of the Russian plain must be discussed separately. Both history and tradition provide definite evidence that the Dniester, the Dnieper, the Don, and the Volga, together with their numerous tributaries (and also the Ural and the Emba), had considerably more water in them during the cold period of the late Middle Ages than they do during the present warm period. Accordingly, navigation conditions were previously much better and there were more fish in the rivers.

The Dnieper and its tributaries are becoming shallower (Mossanovskii, 1886). The water level in the Kal'mius has gone down, and navigation has ceased on this river. Another river, the Lugan', has also become shallow, During the course of the last century the Sambek, which flowed into the Sea of Azov and which was formerly abounding in water, dried up; the vessels of Peter I once dropped anchor in the estuary of this river. Two hundred years ago large warships were able to go down the Don from Voronezh to the Sea of Azov, whereas now this river is navigable only in its lower reaches (Davidov and Tsunts, 1958).

In the not too distant past the Ilovlya was quite full of water and abounded in fish. At present, however, after the spring floods abate, there is not even a continual flow of water in this stream. Formerly, it should be noted, a waterway joining the Volga and the Don led along the Ilovlya. The Medveditsa and the Khoper are also now becoming shallow (Kireev, 1961).

The Volga is at present going through a stage of aging; its channel and the lowlands around its mouth are filling up with sediment (Kel'vin, 1933). The Emba has stopped discharging into the Caspian, and the lower reaches of the Ural have had to be joined to the sea by means of special canals (Fedorovich, 1958).

What is the reason for this deterioration of the hydrological conditions in the southern Russian plain during the present period? Clearly, the conditions became worse because climatic changes entailed a corresponding rearrangement of the global drainage system, changing its orientation to polar. The water-divide line shifted equatorward, the flow gradients of the rivers became less (in accordance with the reduction of the centrifugal forces), and thus the total southward flow became curtailed.

The rivers in question seemed to be "pushed up," and it appeared as if their erosion levels were raised. Thus all of the phenomena typical of

rivers in a stage of senility were observed.

The Danube.

The Danube delta is triangular in shape, with its vertex in the west and its base along the Black Sea. A straight line drawn from the vertex to the sea would be about 80 km long (Figure 12), and the edge of the delta along the sea is about 150 km long. In its delta the Danube divides into three main distributaries: the Chilia, in the north; the Sulina, in the middle; and the Sfantu Gheorghe, in the south.

During the third cold period (late Middle Ages) the river deviated to the right (equatorward), and the main flow was along the southern and middle distributaries. At that time the southern part of the delta advanced more rapidly than the northern part. With the onset of the present period of climatic warming, the Danube began to deviate to the left (poleward), and consequently the bulk of the flow shifted to the Chilia branch, which at present carries about 70% of the total discharge (Zenkovich, 1958).

The rate of advance of the northern part of the delta has increased accordingly. Over the last hundred years this part has moved more than 6 km out into the sea.

The Neman.

In its lower reaches this river has in general a westerly course. The branches of the Neman which supply the most water are: the Atmata, the Skvirite, the Gilge, and the Njemen. During the third cold period (late Middle Ages) the Neman tended to deviate to the left (equatorward), and a considerable part of its discharge was along the southern branch, the Njemen. With the onset of the present period of climatic warming, the river began to deviate to the right (poleward). The part of the delta which is actively expanding at present is thus the region between the mouths of the northern branches, the Skvirite and the Atmata. As Gudelis (1959) notes, the water arteries in the Neman delta are shifting to the north; consequently, some of the southern branches (for example, the Njeman) have lost their connection with the main channel and have embarked upon an independent existence. The general development of the southern part of the delta during the present period is marked by an appreciable amount of stagnation.