Impulses of high river runoff in the centre of the Russian Plain during the Valdai (Weichselian) glacial epoch

Past changes of river discharges may be detected from their traces in river valley morphology as well as from the tendencies of valley development. Indicators of river runoff higher than the present are large palaeochannels in river floodplain and low terraces, including big meanders - macromeanders, and episodes of regional incision of drainage net. The paper describes such phenomena in a number of river valleys where they are found and dated by radiocarbon and OSL methods. Rivers Seym (the middle Dnieper catchment) and Khoper (the middle Don catchment) are taken in the deep incision about 30-35 ka BP (hereafter in calendar years). Low terraces (12-16 m) were formed composed of alluvium dating to 50-80 ka BP. Deepening of river valleys between 35-15 ka BP was found also in the Moskva and Protva Rivers (the Oka catchment). Runoff increase is evident from the concurrent development of incised macrome-anders. Occurrence of higher (20-25 m) terraces within macromeander necks points at possibility of an earlier, probably 60-70 ka BP, start of incision. Low terraces of the Seym and Khoper Rivers are covered by aeolian sands up to 3-4 m thick, which indicates cryoarid environmental settings. Absolute (OSL) dates from aeolian sands range within 20-23 ka BP, i.e., the cryoarid conditions lasted no longer than for several millennia and coincided with the Last Glacial Maximum (LGM). LGM sand covers descend from terraces to the modern floodplain level and is buried by the Holocene overbank fines, which proves the rivers had been entrenched below modern channels already before LGM. Alluviation during LGM produced the "intermediate terrace" only 1-2 m higher than the Holocene floodplain. Between 13-18 ka BP, the last incision occurred accompanied by formation of macromeanders. Active channel migrations produced wide valley floors of southern rivers (the Seym, the Khoper) that now have excessive floodplain width (5-10 km) compared to sizes of modern rivers. Big parts of these floodplains had been formed in the pre-Holocene times and are composed of the Late Glacial alluvium covered by the Holocene overbank fines. In the Moscow Region (the Moskva, the Protva), pre-LGM incised macromeanders continued active development in the Late Glacial. Few neck cut-offs of macrobends occurred between 15-18 ka BP, but most of them had survived throughout the Late Glacial. In the Holocene, river discharges decreased considerably, river channels stabilized and passively inherited the Late Glacial macrobends. The above results evidence that the characteristic feature of the Valdai (Weichselian) epoch was occurrence of high-amplitude alterations of river discharges that had left marked traces in valley morphology and sedimentary record. Time intervals 13-18 and 30-35 ka BP (and probably earlier) were characterized by runoff severalfold as high as its present-day values, and the interval of 20-23 ka BP (LGM) was noticeable for its aridity and high activity of aeolian sedimentation in river valleys.

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