The sequence of sediment accumulation of the first and second terraces of the Tarbagataika River and changes in the stages of incision and filling, as well as sedimentation and soil formation over the last 14 kyr in the river valley were revealed. The dynamics of the sedimentation stages and of the phases of their stabilization and pedogenesis was determined by landscape-climatic changes in the study area. The period of geomorphologic formation of above-floodplain terraces was determined. The floodplain sedimentation regime was changed by the subaerial regime on the second terrace of the Tarbagataika River approximately 8 kyr BP. The accumulation of cover deposits on the first terrace began approximately 3.4 kyr BP. Deposits of the second terrace of the Tarbagataika River were formed during the Late Glacial and the Holocene during the last 14 kyr. They are composed of alluvial sediments that accumulated 14.0–8.0 kyr BP and of cover deposits, which accumulated 8.0–0 kyr BP. The first terrace of the Tarbagataika River of the Holocene age is composed of alluvial deposits, which were accumulated 7.0–3.4 kyr BP, and of deposits of the covering complex, which were formed over the last 3.4 kyr. The phases of intensification of erosion-accumulative processes related to increased water discharge were identified. The main stages of soil formation in the study area were determined. The data we obtained are in good agreement with the previously identified stages of pedogenesis in the Selenga middle mountains and adjacent regions of South-Eastern Siberia and Mongolia.

The influence of anthropogenic and natural factors in the trends and mechanisms of development at various topological levels is determined based on relevant information on the structure and dynamics of fluvial systems in the south of Eastern Siberia in various geodynamic settings. This article considers the current spatial and temporal dynamics of the hydrological conditions of the vast territory of the Angara River and its influence on channel deformations and the manifestation of dangerous processes. An analysis of fluctuations in the maximum runoff using differential integral curves resulted in the identification of six periods of water content according to the maximum annual discharges for the period spanning from the beginning of observations to 2020 for the rivers under consideration. The dynamics and intensity of manifestation of hydrological and geological hazardous processes are demonstrated using a series of studies conducted under various geodynamic conditions. Catastrophic floods brought on by enhanced cyclonic activity are accompanied by the destruction of the bank. The highest rate of bank erosion in the plains is 1.5 to 2 m per year, and for rivers in mountains and piedmonts it is 2 to 6 m per year. An analysis of the dynamics of the development of floodplain–channel complexes in the Upper Angara region makes it possible to distinguish two zones of actively developing floodplain–channel complexes: piedmont and estuarine, separated by a relatively stable plain zone.

The bulletin of Irkutsk State University. Series «Earth Sciences»

The results of studying the specifics of the formation of rain floods in river basins of mountainous areas are presented. The features in the formation of dangerous hydrological events, such as floods and mudflows, are considered. An analysis of the characteristics of rain floods on the rivers of the Eastern Sayan mountains is carried out based on data from the highly detailed monitoring of river flow parameters and atmospheric precipitation using automatic measuring devices. Water level measurements are carried out once every 15/30 min, and precipitation once per hour. The characteristics of floods for the flood season of 2022 on eight rivers are given, as well as floods on the Belaya and Kyngarga rivers from 2019 to 2022. For extreme floods that can cause significant damage, an assessment of flood-forming precipitation is made. The timing of passage, duration, height of the level rise and its amplitude, the rate of rise and fall, etc., are determined. It is established that, during flood season, up to ten rain floods of different sizes occurred on the rivers. There is a synchronicity in the high floods on the left tributaries of the Angara River. Most often, the maximum floods of the year occurred in July. The duration of a flood rise is on average 2–3 times less than its reduction, and the average rate of level rise is three times higher than the rate of reduction. The duration of the highest levels during floods is several hours. For small floods occurring midsummer, it is 15–30 min and is equal to the measurement step of the devices. Flood-forming precipitation during extreme floods can be long-lasting and widespread or intense and short-lived. The total amount of precipitation before the peak of the flood varies from 31.4 to 122.6 mm, and the average intensity of precipitation varies from 0.10 to 1.95 mm/h.

The morphological and morphodynamic features of the Malaya Belaya River in its lower reaches are considered. Various types of floodplain–channel complexes are identified, and some features of their functioning are characterized. The structure of the first terrace deposits in areas of different morphodynamic channel types are analyzed. Analysis shows a significant spatial differentiation of the deposits of the first terrace, associated with the heterogeneity of the morphostructural features of the Malaya Belaya River valley. It is found that the most stable part of the terrace, which has the most ancient age of the stabilization surface, is confined to the areas of development of a relatively rectilinear type of channel. The latter is apparently characteristic of the section of the valley considered and during the formation of the surface of the first terrace. In the meandering section upstream, the height of the terrace decreases slightly. Based on lithologic and facies analysis of sections of terrace and floodplain deposits, data on the rhythm of sedimentation within low morphological levels in the Malaya Belaya River valley are obtained. The main stages of the formation of floodplain generations have been identified. Stabilization of the surface of the first terrace in the areas of a straight channel begins at 10.8 ka. Fixing of the surface of the first terrace at the sites of meandering channel type occurs at 9.3‒89 ka. The formation of a high (up to 3 m) floodplain occurs around 3.5 ka. Low floodplain deposits with a height of up to 2 m begin to form after 2.6 ka. An analysis of the structure of the sediments of the floodplain–channel complexes and the first terrace of the Malaya Belaya River has made it possible to identify two stably developing floodplain–terrace complexes within a small section of the river channel.