In rural areas affected by Chernobyl, accelerated erosion has become a major source of particulate 137Cs in sediment load. The long-term dynamics of the activity concentration in eroded soil material transported from individual slope catchments can be better understood by exploring the 137Cs depth distribution in sediments deposited near cultivated fields. This study focuses on three cultivated slope catchments located in the Chernobyl-affected area of Central Russia. A depth incremental campaign was conducted within zones of sediment accumulation in 2022-2025. The behavior of radiocaesium associated with particles after the Chernobyl accident was controlled by the prompt implementation of remediation measures. Shortly after the accident, the values decreased by more than two times. The radionuclide flux then began to depend on soil erosion processes. Gradually, the thickness of the upper soil that had been eroded became large enough to allow soil material from deeper layers to be involved during ordinary plowing and led to a subsequent decrease in the 137Cs activity concentration. Given the decreasing snowmelt runoff and lack of increase in high-intensity rainfall in the 21st century, the activity concentration of 137Cs in slope runoff has remained quite stable. This phenomenon requires consideration of whether a physically based model for the transport of particulate radionuclides should be developed.

This paper discusses the opportunities and challenges of using vertical distribution of Chernobyl-derived 137Cs in floodplain sediments to evaluate long-term changes of sediment-associated radionuclides in river system. It summarizes the results of more than a decade of research conducted in a lowland river (the Upa River) basin heavily contaminated after the Chernobyl accident. The overbank deposition of sediment-associated 137Cs on floodplains is determined by several factors, which must be considered when interpreting the observed contamination patterns, the most important being topography and hydrological regime, which control frequency of floodplain inundation during high water periods. Frequent floodings are conducive to a better record of 137Cs variations in sediment load. The 137Cs vertical distribution in floodplain sediments can be altered by certain disturbances, which makes its interpretation more difficult. The contribution of local sources to sediment load should be considered to avoid misinterpretation of 137Cs redistribution in upstream catchment. Downstream overbank deposition is impacted by management of river flow through artificial reservoirs, and therefore such floodplain sediments do not represent migration of 137Cs in upstream catchment. Nevertheless, investigation of floodplains downstream can provide valuable data about transport of clay and silt particles carrying pollutants.

This paper explores the use of ¹³⁷Cs derived from Chernobyl as an indicator of sediment supply and transport within small agricultural catchments by analyzing the depth distribution of radionuclides, with a focus on post-Chernobyl changes in the activity concentration of radionuclides. To this end, depth-incremental sampling was carried out along routes of sediment transport within a small agricultural catchment subject to intense radioactive contamination in the Tula region. Some points were set to repeat the position of those made 27 years earlier and to understand the dynamics of deposition and the ¹³⁷Cs content in the sediment load. It has been suggested that a decrease in the activity concentration of ¹³⁷Cs can be used as an indicator of the relative age of deposits. Assuming this, the pattern of erosion product deposition on the sides and bottom of the dry valley was determined. This pattern was found to be stable and consistent with the observed geomorphic features and climate trends: the rate of accumulation in the valley bottom over the past 27 years has dropped almost twice, coinciding with a decrease in snowmelt runoff during springtime and no increase in intense rainfall. Grain-size analysis of the collected samples showed that selective transfer of clay particles may occur, but over a short delivery distance, it is unlikely that the sorting process will significantly alter the downward trend of ¹³⁷Cs concentrations. The proposed approach has the potential to significantly improve the accuracy of sediment budget estimations and environmental quality assessments.

A resampling campaign was carried out in a small agricultural area within the Chernobyl-affected region of the Central Russian Upland to monitor long-term changes in 137Cs contamination and sediment redistribution. Integral soil sampling was carried out on the cultivated slopes, and a depth incremental procedure was used at the bottom of the dry valley. The sampling points ware located in 2024 based on the original study conducted in 1997. The results showed that over the past 27 years, the 137Cs inventory at the studied points had increased on average. The most significant changes occurred in the footslope, where surface runoff concentration occurs. The observed increase in 137Cs inventory may indicate a bias in the assessment associated with the location of points, primarily along the flow lines, where the intensive intra-slope accumulation occurs, in conditions of a tendency to decrease runoff as a result of snowmelt. The 137Cs vertical distribution in sediment redeposited in the dry valley bottom suggests that the sediment and particulate 137Cs transfer from cultivated slopes has decreased since the 1990th, but the pattern of accumulation zones remains unchanged. It is argued that complex re-sampling can serve as an effective tool for assessing the current trends in sediment redistribution but faces with serious limitations.

Khorlakel Lake is located in the central sector of the Greater Caucasus, on its northern macroslope. The closed lake lies at an altitude of 2045.0 m above sea level on a tectonically determined subhorizontal step with a height of 2020–2100 m a.s.l. associated with the frontal part of the thrust. Two cores were drilled, and 17 samples for radiocarbon dating were taken in 2017 in the deepest (≈8 m) part of the lake, which made it possible to create an age model for the range from 8000 to 500 yr BP. To interpret the stratigraphic–temporal sequence of lacustrine sediments, integrated geological and geomorphological studies in the catchments of the lake and Elbashi Creek and in adjacent territories were conducted. It was established that sedimentation in the lake is associated with the influence of the adjacent Elbashi Creek. A number of episodes of proluvial activation over the last 8500 years with the formation of an proluvial fan have been traced. This was followed by lacustrine sedimentation in a dammed lake, the relic of which is Khorlakel Lake. As a result, sedimentation in Khorlakel Lake and in the adjacent territories is clearly divided into two stages with a partition at ≈3 ka BP with the sedimentation of mainly nonorganic material at the first stage and organic material at the second stage. These stages include ten episodes, which are characterized by different sediments features and varying proportions of mineral and organic components. It was found that the initial lake during the Holocene decreased in size due to the uneven growth of the proluvial fan formed by sediments delivered from the catchment of Elbashi Creek. It has been established that some lithostratigraphic boundaries in the bottom sediments of Khorlakel Lake correlate with known strong earthquakes in the Elbrus region, while other boundaries relate to climatic events in the Central Caucasus that occurred during the Holocene. The complete termination of the connection between the current Khorlakel Lake and the Elbashi Creek catchment occurred in the last ≈1 ka BP.

This paper considers the features of accumulation of eroded soil material at the foot of the plowed slope and further to the side of the valley in the presence of a lynchet resulting from mechanical tillage on the lower border of arable land. A small catchment area in the area of intense Chernobyl pollution has been studied, where the position of the lower boundary of the arable land coincides with the upper boundary of the side of the valley over the past few decades. This made it possible to trace the spatial patterns of distribution of eroded material under conditions of stable functioning of the lynchet as an important lithodynamic boundary that accumulates and redistributes the slope runoff of sediments. The volume of material from arable slopes was determined using mathematical modeling of erosion based on a high-precision digital terrain model, climate data, soil erodibility, and crop rotations. The intensity of accumulation was estimated using 137Cs of Chernobyl origin as a reliable tracer. The bulk of the eroded material enters the valley network through a system of slope hollows that concentrate the slope runoff and cut through the valleys. In areas of the valley sides outside the runoff concentration zones, the redeposition of the material generally depends on the morphology of the arable slope, occurs locally, and is associated with episodic overflows as a result of the gradual filling of the depression before the lynchet. Under the existing conditions, the proportion of sediments accumulated on the sides of the valley is relatively low in the total sediment balance of the studied catchment area. Significant changes may occur due to a gradual reduction of the barrier role of the lynchet as a result of accumulation of sediments and climate changes that contribute to an increase in erosion rates.

The transformation of radioactive contamination of agricultural lands with 137Cs isotope is one of the evidences of soil erosion. Changes in the radionuclide content and the corresponding rates of soil loss can be assessed quantitatively by repeated soil sampling at key sites over long time intervals. Being highly labor-consuming, such studies are few in number; they have not been conducted previously in the zone of intense Chernobyl contamination in the Central Russia. The method of repeated sampling (resampling) was used in 2023 within the plowed slopes of a small catchment in the southern part of Tula region, 26 years after the first sampling in 1997. The changes in 137Cs deposits (kBq/m2) that occurred during this period turned out to be statistically significant, with an average reduction of more than 10

Реконструирована история деградации оледенения в бассейне высокогорного озера Сылтранкёль (Приэльбрусье, Северный Кавказ) с конца XIX до конца XX в. В 2022 г. произведено комплексное обследование котловины озера, включавшее отбор отложений и их последующее радиоизотопное датирование, анализ структур, текстур и гранулометрического состава, подсчет варв. Параллельно проанализирована совокупность литературных источников, топографических карт, наземных фотографий, космо- и аэрофотоснимков и собственных полевых наблюдений для определения положения краев ледников на разных временных этапах. Выделены четыре этапа в формировании донных осадков, коррелирующие с состоянием горного оледенения и изменениями конфигурации озерного водосбора. Сходимость результатов, полученных на основании независимых источников, подтверждает высокую методическую ценность исследования донных осадков горных озер как одного из немногочисленных архивов природной среды, фиксирующих изменения оледенения в динамичных природных условиях высокогорий. Deglaciation history of the Syltrankel high-mountain lake basin (Mt. Elbrus region, Northern Caucasus) from the end of the 19th to the end of the 20th centuries was reconstructed. In 2022, a comprehensive examination of the area was carried out, including sampling of the bottom sediments and following radioisotope dating, structural and textural analysis. Simultaneously, a set of published sources; topographic maps; ground, satellite, and aerial photographs; and field observations was analyzed to determine the positions of the edges of glaciers at different times. In the formation of bottom sediments, four stages were identified correlating with the state of mountain glaciation and changes in the configuration of the lake’s drainage area. Converging results obtained on the basis of independent sources indicate the high methodological value of studying bottom sediments of mountain lakes as one of the few environmental archives that record glaciation changes in the dynamic conditions of high mountains.

The intensive pollution of vast areas after the Chernobyl accident, especially in the territories of Ukraine, Russia, and Belarus, has not only become a serious environmental issue, but also presents wide methodological opportunities for studying the functioning of natural systems. The proposed work is a generalization of the results of studies on the migration of 137Cs in the runoff of river sediments, which were carried out in the basin of the Upa River for over 30 years after the accident. This basin is one of the most radioactively contaminated and studied in Central Russia. Over the past three decades, under the conditions of the decreasing snowmelt runoff in the spring and reduced share of cultivated land over the post-Soviet period, the intensity of the 137Cs transfer has decreased. The 137Cs deposit losses associated with erosion activities do not exceed a few percent. Most of the mobilized sediments and sediment-associated radionuclides accumulate in dry valleys or artificial reservoirs. With a general reduction in the durations of floods, rivers have become the predominant channels for the transfer of sediment yield and particulate pollutants. The exploration of the vertical distribution of the 137Cs in the accumulative strata makes it possible to identify the changes in the sediment budgets of the rivers and their radioecological consequences.

Field gamma-sensing under conditions of intense radioactive contamination has shown high productivity in studying sediment-associated pollutants distributed as a result of erosion and accumulation processes. The purpose of this work is to evaluate the applicability of compact gamma detectors without a collimator that narrows the area of gamma radiation registration to determine the vertical distribution of Chernobyl-derived 137Cs. Accumulative strata of sediments formed within the Plavsk radioactive hot spot in the southern part of Tula oblast were chosen as the object of research. The resulting distortions in the estimation of the relative vertical distribution of radionuclides in the soil, limiting the applicability of the proposed measurement scheme, were considered by comparing the obtained vertical distribution of the gamma quantum counting rate with the actual distribution of 137Cs deposits. The main prospects for further application of gamma-sensing of the soil cover at relatively high concentrations of radionuclides in accumulated sediments were identified.

The Plava River basin (Tula region) is considered the most polluted with Chernobyl fallout region in Russian Federation. Detailed studies of sediment redistribution and migration of 137Cs produced by Chernobyl were conducted in the basin. The article provides an overview of results from various studies conducted during the last 30 years estimating the rate of erosion and sediment redistribution based on different methodology including 137Cs. Sediment budgets for different parts of the fluvial network were developed based on estimated rates of erosion and accumulation of sediments within the Plava River basin for the post-Chernobyl period. It has been established that eroded from agricultural fields sediments accumulate on the slopes of interfluves (38.4%) and in the bottoms of dry valleys (2738%). Part of the sediment delivered by slope runoff and temporary watercourses from the slopes of the interfluves to the bottoms of river valleys was redeposited on river floodplains (1011%), and the remaining part (13.124.7%) passed into the river sediment load. The value of basin contribution of sediments to the Plava River sediment runoff is somewhat overestimated due to the specifics of the erosion models used to calculate soil erosion from arable land. It has been established that for the period of more than 25 years since the Chernobyl accident, 5% of 137Cs deposits were lost due to soil erosion (less than 0.2% annually). According to evaluated sediment budget, only a quarter of mobilized material leaves basin as a sediment yield. Thus, only about 1% of the initial 137Cs fallout was removed from the Plava basin. The perspective directions for research on sediment redistribution in the plain river basins located in the temperate zone using 137Cs as a technogenic tracer are outlined.

Abstract—Flash floods are one of the most dangerous hydrometeorological events all over the World. In the current paper stochastic parameters of flash floods formation are studied on the basis of data on flash floods in 1990–2021 in the small river basins of the Caucasus and Crimea Black Sea Coast. The main factor of flash floods formation is heavy rain, but in some cases its occurrence could depend on critical combination of various factors. Flash floods are usually formed in summer-autumn period in the studied region with the maximum of observed events in August. They are characterised by very rapid water level rise of about 1.2–1.3 m/h. Sediment yield during one flash flood could be compared with mean annual values. Statistical analysis of precipitation long ranges demonstrates probability of more often flash floods occurrence in the region in comparison with observed events.

The change in sediment yield is an important indicator of the natural environment dynamics, depending on the combination of landscape, tectonic and climatic conditions. Assessment of sediment yield often based on the results of studying the bottom sediments of mountain lakes with relatively compact catchments. However, for correct reconstructions, in addition to analyzing lake sediments, it is necessary to study the causes and mechanizm of sediment redistribution in their catchments, to identify sediment delivery pathways to the reservoir and their possible changes over different time windows. The drainless Lake Khorlakel, located at the altitude of 2045.0 m above sea level on the northern macroslope of the Greater Caucasus. It is a suitable testing ground for complex research: on the one hand, the relict reservoir is an ideal sedimentation trap, and on the other, it is located with in an area of intensive exogenous processes and tectonic activity. The two boreholes were drilled in 2017 in the deepest (≈8 m) part of the lake and 17 samples collected taken from the cores for radiocarbon dating, which made it possible to build an age model for the range from 8000 to 500 yr. BP. Complex geological and geomorphological studies were carried out in 2021 to interpret the obtained data. It was found that sedimentation in the lake is associated with runoff and sediment redistribution in the Elbashi creek catchment. A number of episodes of proluvial activation with the formation of an outflow cone, followed by lake accumulation, have been traced for the last 8 kyr. The connection between lake and catchment ceased only in the last 1 kyr. BP. Two main stages of lake sedimentation with a boundary of 3 kyr. BP and 10 episodes, that are characterized by different proportions of mineral and organic components in bottom sediments were established. Some of the lithostratigraphic boundaries correlate with strong earthquakes that occurred in the Elbrus region, and some – with climatic events.

The purposes of this study are to determine the content and origin of anthropogenic fallout radionuclides (FRN) in soils of Mount Khuko, located in the western sector of the Caucasus Mountains and to assess the possibility to use them for evaluation of sediment redistribution for the alpine grasslands,. The field study was carried out in August 2019 near the top of Mount Khuko, located in the western part of the main Caucasus Mountain Ridge. Integral and incremental soil samples were collected from the different morphological units of the studied area. The content of Cs-137 and Am-241 in soil samples was evaluated using laboratory gamma-spectrometry. A part of samples was selected for Pu isotopes extraction and then alpha spectrometric analysis. It was established that the Cs-137 contamination of soils in the studied area has at least two sources of origin. The first source is the Cs-137 bomb-derived fallout after the bomb tests in 1950-60th, which is widespread across the globe. The second source is Cs-137 Chernobyl-derived fallout High random variability (Cv = 25-42%) was found within reference sites, located at the undisturbed areas on the local flat interfluves due to high variability of soil characteristics (grain size, density, organic matter content etc.). However minimum spatial variability (range 12,2-14,3 kBq/m(2)) was identified for the mean value of Cs-137 inventories for all 5 reference sites located in the different parts of the studied area. It is difficult to separate individual peaks of the bomb-derived and Chernobyl-derived Cs-137 falloutin sediment sinks with low sedimentation rates. Application Pu-239,Pu-240 as an additional chronological marker allows to identify the origin of above mention peaks in the soils of alpine grasslands and of dry lake bottom.

Verification of soil erosion models (WATEM/SEDEM for rainfall and tillage soil erosion and regional model of the State Hydrological Institute for snowmelt soil erosion) was carried out on the basis of estimates of soil erosion and deposition rates in an arable catchment of dry valley in the Vorobzha River basin located in the central part of the Srednerusskaya Upland in the forest-steppe zone. The rates of soil loss/gain for the entire period of agricultural use were obtained using the soil truncation method. The rate of sediment deposition in the dry valley bottom for the period after the 1986 was determined based on interpretation the Chernobyl-derived 137 Cs depth distribution in the sediments. Comparison of the model calculations with field-based method estimates was generally in good agreement. Also, the erosion model calculation satisfactorily reflected the pattern of zones with different intensity of soil erosion. WATEM/SEDEM made it possible to take into account the movement of soil matter by tillage erosion. Some discrepancies between the modeling results and field data were mainly due to the insufficiently detailed input parameters, e.g., the data on crop rotations or local microrelief specifics. WATEM/SEDEM did not accurately estimate the rate and volume of sediment accumulation, especially for areas beyond cropland.

Variations of 137Cs   content in suspended sediments during flood periods along the river valley bottom and in time since 1986 are revealed based on the analysis in low floodplain deposits and hydrological regime of the Upa River in the river reach between place of the Plava River confluence and the Upa river mouth. It is established that for the period 1986–2014 years there was a decrease in the proportion of sediment and sediment-associated 137Cs  of basin origin in the river sediment yield, especially noticeable after 2006. Reduction the 137Cs  content in floodplain sediments along the Upa river on the reach from the confluence with the river Plava, which are drained the most radioactively contaminated part of the river Upa basin, and up to the mouth is synchronically change with river water discharge as the catchment area increases. Contemporary 137Cs   inventory in sediments of the low floodplain of the Upa river exceed the initial levels of its contamination after the initial fallout of Chernobyl-derived 137Cs . The increase of inventory is associated with the accumulation of contaminated sediments at a rate of 1.5–2.7 cm/yr. during floods.

Medium magnitude debris flow phenomena are widespread in the Kola Peninsula Mountains. Most frequently observed types are snowmelt period slushflows and rainfall-induced debris flows. Similar sets of hazardous events are reported for mountainous areas of Scandinavia, Japan, and Northern America.