During the third cold period (late Middle Ages) the Edsin Gol deviated to the right (equatorward). Consequently, the main discharge was along its right-hand branch, the Kara-Baishingen Gol, following a northeasterly course to the vast Khadan-Khoshu depression. It was at this time that the large medieval city of Kara Khoto existed on the banks of this branch of the river. Lakes Gashun Nur and Sogo Nur must presumably have had low levels during this period, if indeed they were supplied with water to any extent at all.
During the third (present) warm period, this river has tended to deviate to the left (poleward). Thus, the discharge of the Edsin Gol gradually became transferred to its left-hand distributaries, the Ikhe Gol and the Morin Gol.
The Morin Gol branch, which has a north-meridional course, and also its standing-water bodies Gashun Nur, thus now have more water in them. The Ikhe Gol branch, and its standing-water body Sogo Nur, on the other hand, which are situated more to the south, have gradually become shallower. Finally, the southernmost branch, the Kara-Baishingen Gol, has dried up.
These changes in the river were the main reason for the downfall of the city of Kara Khoto. As pointed out by Murzaev (1956), Kara Khoto and the towns of the Takla Makan clearly went to ruin because the river branches or the main channel of the river changed course, due to natural causes. Uzin (1958) writes that the Edsin Gol previously flowed quite close to Kara Khoto, rather than tens of kilometers away from it as it does now, and that the branches of the river flowed on both sides of the city, giving life to it. However, certain changes took place, causing the river to turn toward the northwest and producing the Sogo Nur and Gashun Nur lakes.
During the second warm period (early Middle Ages) the Tarim and the Konche Dar'ya deviated to the left (poleward); the two rivers combined to some extent and discharged along the southern foothills of the Kuruk Tagh range, forming Lake Lop Nor in the northeastern part of the Lop Nor depression. At this time the northern edge of the Takla Makan Desert
was more abundantly watered than the southern edge. Apparently this explains the decreased number of settlements in the southern part of the Tarim plain, for example in the vicinity of the Keriya River (Sinitsyn, 1959). It also explains why the east-west trade route went through the northern part of the plain during the early Middle Ages.
In the third cold period (late Middle Ages) the rivers deviated to the right (equatorward). Accordingly, Lake Lop Nor was shifted to the southwest (by about 150 to 200 km), to the foot of the Astin Tagh. The east-west trade route also moved from the northern part of the Tarim plain to the southern part, since conditions there were better at that time.
During the third (present) warm period the Tarim and the Konche Dar'ya show a definite tendency toward leftward (poleward) deviations.
The present-day Lake Lop Nor has neither a constant position nor a definite size. Sinitsyn (1959) reports that in the seventh century B.C., according to Chinese sources, Lop Nor was situated in the northern part of the ancient lake basin, at the mouth of the Konche Dar'ya. Then, for a thousand years (or perhaps somewhat less), the lake was located to the south, in the Kara Hoshun region, at the mouth of the Tarim. At the beginning of the present century the southern lake dried up.
Thus, the change in position of the lake in the Lop Nor depression, and also the periodic drying up of this lake, are caused by redistributions of the water along the channels of the Tarim and the Konche Darya. Present day observations (see "Zarubezhnaya Azia" [Non-Soviet Asia], edited by B. F. Dobrynin and E. M. Murzaev, 1956, p. 269) indicate that now one branch of the Tarim joins with the Konche Dar'ya to flow along the ancient Kum Dar'ya channel; after 250 km this water flows into Lop Nor, and as a result of this the lake has moved northward. The studies of Chao and Chao (1961, p. 88) also show that at present the channels of the Tarim have a tendency to migrate toward the north.