Abstract: The paper presents a combined analysis of the lithological, granulometric and mineralogical composition of unconsolidated sediments in the Sentsa River basin (Oka Plateau, Eastern Sayan, Russia). Complex analytical studies and drilling operations were aimed at studying the lake, river and moraine sediments that fill the river valley in order to reconstruct the conditions of their accumulation. Granulometric analysis, performed using sieve and laser diffraction methods, made it possible to establish the sequence of sediment accumulation in this dammed paleoreservoir and the position of the paleotable of the permafrost (at depths of 1.2 and 2.1 m). The mineralogical composition of alluvial sands confirmed the predominantly quiet water regime of their accumulation (in lake and lake-marsh conditions). The composition of moraine sediments and the structure of moraine ridges indicate the activity of water flows during the melting of the glacier, which formed a ‶layer cake″ of a terminal moraine ridge, as well as a stadial moraine with a thick sandy layer (more than 3.0 m) in the middle stream of the Sentsa River. Deep drilling allowed to establish for the first time the structure of the vertical section of the river valley and the permafrost thickness, which amounted to 44 m. A borehole with a depth of 45.1 m penetrated a section composed of frozen silts, clayey silts, clays and sands, as well as sub-permafrost groundwater. The cryotextures of unconsolidated sediments are predominantly massive and rare schlieren, with lenses and layers of ice up to 8-10 cm thick at different depths. Radiocarbon dating of a core sample taken from a depth of 1.5 m showed an age of 3510±120 years and confirmed the existence of an open water reservoir in the late Holocene in the Sentsa River valley.