Millennial- and multi-centennial scale climate variability during the Holocene has been well documented, but its impact on the distribution and timing of extreme river floods has yet to be established. Here we present a meta-analysis of more than 2000 radiometrically dated flood units to reconstruct centennial-scale Holocene flood episodes in Europe and North Africa. Our data analysis shows a general increase in flood frequency after 5000 cal. yr BP consistent with a weakening in zonal circulation over the second half of the Holocene, and with an increase in winter insolation. Multi-centennial length phases of flooding in UK and central Europe correspond with periods of minimum solar irradiance, with a clear trend of increasing flood frequency over the last 1000 years. Western Mediterranean regions show synchrony of flood episodes associated with negative phases of the North Atlantic Oscillation that are out-of-phase with those evident within the eastern Mediterranean. This long-term flood record reveals complex but geographically highly interconnected climate-flood relationships, and provides a new framework to understand likely future spatial changes of flood frequency.

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<p>Large meandering palaeochannels (macromeanders) are found in the river valleys of the East European Plain. Channel parameters of such macromeanders are few times larger than the recent ones of the same rivers. Such macromeanders are widespread in the temperate climate zone of the Northern Hemisphere. Formation of these macromeanders is usually associated with Late Glacial (the end of MIS 2). The main aim of our study was the reconstruction of the river valleys’ development of the Central part of the East European Plain and establishing of the absolute chronology of the large paleochannels formation in this region. Few key sites were studied in the river valleys of the Central part of the East European Plain. Hand and mechanical coring, examination of natural and human-made exposures, sampling for different types of analysis, geomorphological discriptions and topographic profiling were done during the fieldworks on the key sites. Radiocarbon and OSL dating, grain-size and spore-pollen analysis were done in the laboratories.</p><p>Data analysis shows the following results and conclusions. The alternation of high and low river runoff was typical for the valleys of the study region in the end of the Late Pleistocene. This led to the alternation of river incision and aggradation in the valleys. About 30-35(40) ka ago there was epoch of rivers incision. High runoff is proposed as the reason of this incision. Before LGM rivers had already been incised down deeper than the modern levels. LGM time (20-23 ka ago) was characterized by cryoaridic conditions, low runoff and accumulation in the river valleys. Not only alluvial, but also aeolian accumulation in the river valleys was characteristic for this period. In that time aeolian covers and aprons few meters thick were formed in river valleys (mostly on the terraces’ surfaces). After LGM the high runoff epoch started, which was dated 12-19 ka ago. In that time large meandering palaeochannels (macromeanders) and modern wide high floodplains were formed in the river valleys. These macromeanders were formed under conditions of extremely high spring discharges in severe climate conditions. Our data shows that this epoch of high runoff was interrupted by a short low runoff period (~15-16 ka ago). So, the Late Pleistocene epoch of high floods and large paleochannels formation had two periods – about 17-19 ka and 12-15 ka ago. Runoff in the Holocene became lower than in the Late Pleistocene, channel parameters decreased.</p>

<p>The Moksha River valley was studied in its lower part between the Tsna River confluence and the mouth of the Moksha River. Wide floodplain and two levels of terraces are presented on the studied part of the valley. The height of the floodplain is from 1 to 6 m, of the first terrace – about 9-11 m, of the second terrace – 18-22 m. The width of the valley in this area is about 14-16 km, but sometimes it can reach 20-22 km and more. The width of the floodplain is about 12-14 km.</p><p>The Moksha River is a meandering channel. Large and small (modern-size) meandering palaeochannels spread widely on the floodplain surface. These palaeochannels were the main objects of our study. Small palaeochannels have the same parameters as the modern river channel: their width is about 100-150 m, wavelength is between 300-400 and 600-700 m. For the large palaeochannels (macromeanders) the mean parameters are the following: width is about 250-300 m, wavelength is about 1500-2000 m. These large palaeochannels are the signs of high flood activity epoch(s).</p><p>In our study we used a number of field and laboratory methods. Twelve boreholes in large and small palaeochannels were made during fieldwork in August-September 2019. Organic material from studied palaeochennels was sampled to make radiocarbon (AMS) dating to find the time of palaeochannels’ formation and infilling. Also we made the reconstructions of paleo-discharges of the Moksha River based on paleochannels’ parameters.</p><p>We studied both large and small palaeochannels to reconstruct palaeohydrology and history of the Moksha River valley development in Late Pleistocene. Large palaeochannels correspond to the time of high river runoff. The oldest ones of small palaeochannels were studied to know the time of lowering of the river runoff. Presumably, large palaeochannels were formed at the end of Late Glacial (after LGM) when river runoff was much higher than the modern one. This period of extremely high runoff was previously distinguished in many river valleys of East European Plain, where formation of large paleochannels is usually associated with Late Glacial (the end of MIS 2). Lowering of runoff on the central part of the East European Plain is usually associated with the beginning of the Holocene.</p><p>This study is supported by Russian Science Foundation (Project № 19-17-00215).</p>