This article presents the results of a comprehensive geomorphological and paleogeographic study of the central part of the Oka Plateau (Eastern Sayan). The relief structure of the region is characterized in detail. A special place in it is occupied by elements of the paleohydronetwork: basins of drained moraine-dammed water bodies and abandoned fragments of valleys, including marginal channel. A comprehensive record has been obtained from the bottom sediments of Lake Sagan-Nur, which makes it possible to reconstruct a consistent picture of changes in the natural environment in the lake basin in the Holocene. It is shown that, about 8600-7100 BP, the lake was fed by glaciers, large flood events occurred in its basin, the water level in the lake was higher than today, and it was probably connected to a moraine-dammed paleowater body (traces of which are recorded in the estuary part of the Aynak River valley). Later, about 7100-5500 BP, a sharp drop in the rate of lacustrine sedimentation and a gradual attenuation of the relative amplitude of flood events occurred. After 5500 BP, the warming of the regional climate, especially in winter, and the disappearance of the remnants of the late Pleistocene glaciers caused a gradual expansion of dry areas at lower hypsometric levels favorable for the regional development of pine forests and the rise of the upper boundary of pine on the slopes. At the end of this time interval, the last restructuring of the local hydrographic network occurred and the modern river system was finally formed. Reconstructions for the last 5500 years show a much calmer hydrological regime and sedimentation pattern. Palynological indices also suggest a warming of the regional climate and a decrease in soil moisture, which took place following a change in the influx of solar radiation in the temperate latitudes of the Northern Hemisphere.

The Central part of the Oka Plateau lying in the East Sayan Mountains is still a poorly studied area of southern Siberia as regards its paleogeography. This gap can be partially replenished by the results of the present study. This study is focused on reconstruction of the central Oka Plateau environment in the Middle-Late Holocene. The pollen from bottom sediments of Sagan-Nur Lake provided a qualitative reconstruction of the vegetation in the catchment area of the lake as well as the quantitative reconstruction of dominant vegetation types obtained via the biomization method. The reconstruction suggests the dominance of the tundra vegetation consisting of dwarf birch, alder, and willow with patches of spruce and larch between about 8120 and 7000 cal. yr BP. The climate was sharp continental with high soil moisture resulting from summer permafrost thaw. The expansion of the forest biome began in the Central Oka Plateau at about 7000 cal. yr BP due to climate warming, hydrological network reconstruction resulting from complete thaw of regional glaciers and degradation of the permafrost rocks. Around 3200 cal. yr BP, the larch forests with the participation of Siberian pine started spreading across the Sagan-Nur Lake catchment area, thus suggesting colder conditions than before. The obtained reconstructions can help identifying the promising lakes and their catchment areas in the East Sayan Mountains for potential sustainable development through special projects (e.g., educational, tourist, environmentally protected).