At the end of the Pleniglacial to the first half of the Late Glacial, approximately 14,000–18,000 years ago, river channels on the plains of Northern Eurasia were up to 10–15 times larger than modern river channels in the same basins. Fragments of these large meandering paleochannels are widespread on river floodplains and low terraces. The hydrological regime of these rivers is of great interest from a paleoclimatological perspective. Morphometric characteristics of large paleochannels – channel width and meander wavelength – were measured on topographic maps and satellite images. Morphometric relationships between modern channel widths and average maximum water discharges were established. These relationships were used to reconstruct maximum discharges during floods for rivers in the Dnieper, Don, and Volga basins and the rivers in Western Siberia. The daily runoff depth at the flood maximum, which corresponds to the maximum depth of daily snowmelt during the snowmelt period, is normalized to unit river basins with an area of <1000 km2. The average value for the southern megaslope of the East European Plain was 50 mm/day (six times the modern value), including 50.6 mm/day (six times) for the Volga River basin, 50.7 mm/ day (seven times) for the Don River basin, and 48 mm/day (10 times the modern value) for the Dnieper River basin. For the rivers of the north of the West Siberian Lowland, the average maximum daily runoff depth was 64 mm (2.5 times the modern value) and 54 mm in the Ob River basin (8 times the modern value). A paleohydrological analogy was used to convert these maximum runoff values into annual averages. The territories with the modern analogues for the climatic conditions of the late Pleniglacial – Lateglacial were determined on the basis of the paleofloristic method and according to data from mathematical climate modelling. Calculations revealed that on the southern megaslope of the East European Plain and on the West Siberian Lowland, over an area of 5.52 million km2, average snow water reserves before snowmelt were 309-390 mm, while flood runoff from this area, assumed to be close to the annual runoff volume, was 1706-2150 km3, which is 2.1-2.7 times greater than the modern value. These results contradict the established notion, based on the xerophilous vegetation, that the periglacial climate on the plains of Northern Eurasia was generally dry. The results of paleohydrological reconstructions showed that under the conditions of a periglacial climate, there was a sharp seasonality in the intra-annual distribution of precipitation; atmospheric precipitation in the form of snow covered the surface of river catchments during a long winter; spring snowmelt was short-lived and intense; floods were short with a high maximum. In summer, there was little precipitation and runoff from permafrost-covered areas was insignificant, which contributed to the development of xerophilous vegetation.