ИСТИНА

The Arctic territory in Siberia and Alaska is characterized by the presence in river valleys of numerous local traces of a decrease in river discharges in the recent past. The main ones include: 1) the presence of fragments of ancient ox-bows on the floodplains with a meander wavelength and channel width greater than the modern ones; 2) the narrow modern channel at the bottom of large ancient meandering river and the presence of a low floodplains in the bottoms of such large bends with the width, which corresponds to the large bend wavelength; 3) the chains of ponds in narrow modern channel, which reflect a former existence of the large channel. On the Yamal Peninsula (the West Siberia, Russia), all these signs of a decrease in river discharges are characteristic for in several river basins, and the secondary cryogenic relief on fluvial relief surfaces indicates that they originated in the relatively recent past (200 - 1000 years ago). One area with wellpreserved signs of reduced runoff have been identified in the Mordy-Yakha River basin. In the upper reaches of the river the channel width and meander wavelength decreased about 1.5 times in comparison with the ancient channel width in the ox-bows, in the middle reaches – more than 2 times and in the lower reaches of the river – by approximately three times. The calculations with the use of the relationship between the channel width and the mean maximum discharge shows that the latter has decreased in the lower part of the river basin by approximately 4.5 times. This reduction in maximum discharges nearly completely ceased the floodplain inundation during the spring flood, which allowed to use the territory of the floodplain of the Mordy-Yakha River for the construction of infrastructure facilities of Bovanenkovskoye gas field. The second area cover the river basins of the southeastern Yamal, where the Novoportovskoye oil and gas condensate field is located. The sizes of meander wavelengths of the rivers do not changed significantly, following the regional relationships with the basin area. The widths of the channels at the lower reaches also follow the regional relationships and the meander wavelength/width ration is not more than 10 for these reaches. In the upper reaches of these rivers the channels widths decrease dramatically, so that the meander wavelength/width ration is about 50. These narrow channels bear all signs of their width decrease, being shaped by the chains of ponds and are bounded by low floodplain, which reflect the size of the former channel width. Examples of this kind can be found throughout the Arctic. Such river basins do not have territorial development. They are local, neighbouring basins often bear no signs of a decrease in river flow in the recent past. In this respect, this stage differs from the well-studied period at the boundary of the Late Glacial – the Holocene, when a decrease in flow affected river basins on all lowlands of the Northern Hemisphere. Keywords: River channel morphology, Hydrological indicators, Safety of infrastructure facilities