Networks of dry valleys (or balkas) and hollows in the upper reaches of fluvial basins in extraglacial areas in the Penultimate Glaciation (Marine Isotope Stage 6—MIS 6) regions of the East European Plain demonstrate clear incision/aggradation rhythms corresponding to global glacial/interglacial climate cycles. The first phase of each incision/aggradation rhythm began after the global glacial maximum and was characterized by a cool and humid climate, permafrost and sparse vegetation, when high surface runoff and active linear erosion formed a dense network of gullies. The second phase occurred at the glacial–interglacial transition and the subsequent interglacial period with its warm and humid climate and dense vegetation. This phase was distinguished by the partial filling of fluvial forms with slopewash deposits, the transformation of gullies into dry valleys (balkas) and the subsequent stabilization of fluvial forms marked by the formation of mature soils on the sides and bottoms of balkas. The third phase of the rapid accumulation of balkas developed during the cold and dry part of the next glacial epoch, resulting in the balkas becoming shallow hollows filled in with sediments. The last full incision/aggradation rhythm occurred in the late MIS 6 to mid-MIS 2. The erosion network formed during the late MIS 6 was almost completely filled by mid-MIS 2, and its manifestation in the modern topography is limited to a network of shallow hollows in the upper parts of the fluvial systems. The modern (incomplete) incision/aggradation rhythm began in the late MIS 2 and caused the formation of the modern erosion landscape in the upper reaches of fluvial systems. This rhythm is now in the stabilization phase, and the main accumulation phase of this rhythm is still far in the future.

Между параметрами структуры современных речных систем и их гидрологическими характеристиками существуют как косвенные, так и прямые связи. Такие связи используются для оценки гидрологических характеристик древних русловых систем по параметрам их структуры в случае совпадения ландшафтно-климатических и литолого-геоморфологических условий на современных и древних водосборах. Эрозионный рельеф Русской равнины сохраняет информацию о прошлых событиях максимального стока воды. Максимальный сток во время валдайского позднеледниковья оставил в рельефе фрагменты больших меандрирующих пале-орусел в речных долинах и сеть балок на водосборах. На склонах водосборов широко распространена и хорошо сохранилась сеть ложбин, многие из которых сформировались во время события максимального стока воды в период деградации московского покровного оледенения. Густота сети балок и ложбин является индикатором гидрологических условий их формирования, в первую очередь максимального стока. Применены два способа оценки этих гидрологических условий: 1) распространение на прошлое современных статистических регрессионных связей густоты речной и долинно-балочной сети областей распространения вечной мерзлоты с величинами стока воды; 2) математическое моделирование формирования эрозионного рельефа в верхних частях водосбора с поиском таких гидрологических характеристик стока, которые соответствуют фактическому эрозионному рельефу. Первым способом получены значения годового слоя стока воды для всего бассейна р. Хопер для валдайского позднеледниковья 200-250 мм, для московского позднеледниковья 600-700 мм. Вторым способом для небольшого водосбора балки Переполье в средней части бассейна Хопра получены величины 230 и 690 мм соответственно. В поздневалдайское время формирование эрозионного рельефа происходило при стоке в 2-3 раза, а во время московского позднеледниковья - в 6-7 раз больше современного.

The headwaters of fluvial systems on the East European Plain between the boundaries of the Marine Isotope Stage 2 (MIS 2) and MIS 6 glaciations evolved during the last 150,000 years. At least three main events of high surface runoff caused intensive erosion: at the end of MIS 6, at the end of MIS 2 and in the Middle Holocene. Erosion developed in the territory with variable resistance of geological substrate, from hard-to-erode tills to weak sandy deposits. All erosional features in moraines formed in the pre-Holocene time. Even relatively large forms, such as balkas (small dry valleys), have not yet reached concave longitudinal profiles. A general tendency of their development was deepening. Short episodes of incision occurring during climatic events with increased water flow alternated with long periods of stabilization. Sand-covered areas are most favorable for linear erosion. The gullies formed in the Middle Holocene developed concave longitudinal profiles. The diversity of catchment areas, initial slope inclinations and sediment properties causing their resistance to erosion led to greater differences in the relief features and evolution of the upper reaches of the fluvial systems within the MIS 6 glaciation area compared to the more uniform landscape conditions in the extraglacial regions.

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The study is based on the results of multibeam echo-sounding and high-frequency seismic profiling during the 2018-2022 cruises of the R/V “Akademik Nikolai Strakhov” and “Akademik Boris Petrov”. Regular changes of morphometric parameters and the internal structure of pingo-like formations of the shelves of the Pechora and Kara seas were revealed. A morphometric analysis of pingo-like formations was carried out, which made it possible to draw conclusions about their relative age, as well as the role of near-bottom currents and slope processes in their modern dynamics. It was found that the density and morphological variety of pingo-like formations depend on the geological and tectonic features of the bottom area, the presence and nature of permafrost, the intensity of degassing, and the time of shelf flooding during the Holocene transgression. Pingo-like formations on the shelf, where the depth exceeds 70-80 m, emerged at the early stages of the Holocene transgression, and by now the permafrost there has largely thawed out. At the same time, pingo-like formations are still prominent in the relief and actively transformed by bottom currents, slope and, possibly, pseudovolcanic processes associated with ongoing degassing. Pingo-like formations are rare within shallow (up to 20-30 m) shelf areas close to the shore, and, apparently, continue their evolution at present. At the same time, the large thickness and continuity of permafrost prevent active fluid flow, acting as a seal. Pingo-like formations in the shallow-water zones are mainly cone-shaped mounds without intensive degassing. The density of pingo-like formations is maximum at the intermediate depths (from 20-30 to 70-80 m), in the presence of insular or discontinuous permafrost, under high fluid flow intensity within the fault zones and oil- and gas-bearing structures. Near-surface sediments in such areas are characterized by a combination of localized processes of heaving and active degassing. It predetermines a wide variety of the morphological types of pingo-like formations.Keywords: Arctic, bottom topography, degassing, pingo-like features, acoustic anomalies, multibeam echo sounding, seismoacoustic profiling