Middle Holocene Decline of River Runoff in the Volga Basin and Its Effect on the Caspian Sea Level

Research in quantitative paleohydrology shows that a phase of very high river discharge on the northern Eurasian plains in the late Pleniglacial (about 18–14 ka BP) was followed by a much less studied phase of reduced discharge and small-channel formation. To characterize this morphohydrological metamorphosis, we investigated the morphometry of small meandering paleochannels on floodplains at 132 river-valley sites in the Volga basin. On average, the widths and meander wavelengths of these paleochannels are approximately half those of modern channels. The ages of the paleochannels range from 10 to 4 ka BP. The hydrological regime of small mid-Holocene meandering rivers was reconstructed using morphometric relationships, principally the power-law relationship between bankfull channel width and mean maximum flood discharge and the relationship between maximum discharge and contributing basin area. The mean reconstructed daily maximum runoff depth for small catchments in the Volga basin during the snowmelt period in the mid-Holocene was about 5 mm/day, approximately half the modern value. The ratios between mean annual and mean maximum runoff depths in the mid-Holocene were estimated from modern regional analogues of the ancient climate, using climate models for the 6 ka BP time slice. Annual river runoff depths in the Volga basin in the mid-Holocene were then calculated from these analogue ratios and the daily maximum runoff depths. The mean annual runoff depth in small catchments was about 100 mm, almost half the current runoff in the Volga River basin. The annual Volga River runoff volume in the mid-Holocene was approximately 132 km3, compared with the current value of 250 km3. This decline in river runoff in the Volga basin resulted in a low stand of the Caspian Sea in the early–middle Holocene, with a calculated potential sea level of −65 to −79 m abs.