МОРФОГЕНЕТИЧЕСКИЕ ТИПЫ И ДИНАМИКА РЕЧНЫХ ДЕЛЬТ ГОРНЫХ ОЗЕР

Abstract

Based on the analysis of published sources, topographic maps, and satellite images, new data were obtained on the hydrological regime, morphology, size and dynamics of river deltas in the lakes inthe mountains of southern Russia and Mongolia. For high mountain lakes (Baikal, Issyk-Kul, Son-Kul, Chatyr-Kul, Sevan, lakes of Mongolia), most small rivers form alluvial fans with numerous channels. Large rivers flowing into the valley bays form multi-arm lobed padded deltas (Issyk-Kul Lake). Less common are blocked estuaries, deltas filling estuarine lagoons and prograding deltas on the open coasts (Lake Baikal, some lakes of Mongolia). The most studied are the river deltas flowing into Lake Baikal and Issyk-Kul. During the Pleistocene, River Selenga formed three alluvial fans with a total area of 1.8 thousand km 2 : 1) the Middle Pleistocene fan at 12–13 m above the level of Lake Baikal (457 m a.s.l.); 2) the Late Pleistocene fan at 8–12 m, and 3) the Holocene fan at 0.5–2 m. Over the past 78 thousand years, the Selenga River has prograded its delta by about 20 km into the lake. For the period 1980–2013 no delta growth has been observed. Lake Issyk-Kuls level fluctuations had a strong influence on the formation of the hydrographic network of the Issyk-Kul basin and on the dynamics of river mouths. Deep regression at the end of the Upper Pleistocene led to a lake area reduction by 2000 km 2 , incision of the rivers of the Issyk-Kul basin into lacustrine shelf deposits and the development of erosive river valleys along the entire perimeter of the lake, preserved to date in the form of flooded underwater valleys. In the recent epoch, most of the rivers of the Issyk-Kul basin have cut through mouth estuaries and over the past 150 years have formed various morphodynamic types of deltas (from the open-coast alluvial fans to multi-branch deltas filling ingression gulfs). One to three km elongation of the high-discharge rivers occurred during this period. The increase in the area of river deltas over the past 30 years (the total effect of the decrease of the lake level and accumulation of alluvium) ranged from 0.01 to 0.07 km 2 . Several large rivers of Russia (Upper Angara, Selenga) and Mongolia (Teysin-Gol, KobdoGol) have delta areas from 200 to 500 km 2 , the remaining deltas do not exceed 50 km 2 . The current progradaion rate of most lake deltas over the past 30 years is very low due to an extremely insignificant load of suspended sediment, with the exception of some large rivers.