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In 2017, a spillover dam was constructed in the middle course of the Amazar River of Russia, forming a reservoir to provide water to the Amazar Pulp and Saw Mill project known as Polyarnaya. The dam uses an integrated approach, combining hydrochemical, hydrobiological, and ichthyological methods, as well as echo sounding. Comprehensive studies of the transformation of the Amazar River into a reservoir demonstrate the initially low biodiversity of hydrobionts characteristics for a semi-mountain river under the conditions of the sharply continental climate of the Trans-Baikal region. During the initial stage of formation, the reservoir was similar to the original watercourses in physical and chemical parameters and in the composition of the flora and fauna. It featured extensive shoals that were gradually turning into silt-covered and plant-filled shallow bays. These bays will eventually be locations of maximum concentration and diversity of hydrobionts and future nursery and spawning grounds for fish. The construction of the dam has significantly changed the hydrology of the Amazar River downstream of the dam. These findings reveal problems related to fish migration to the Thymallidae and Salmonidae spawning areas, as well as reductions in the biodiversity and quantity of the macrozoobenthos typical for run-of-river reservoirs.

Morphological signs (32 plastic and 7 meristic) and biological parameters (size, weight, age and sexual structure, fecundity) are considered for weather loaches of gen. Misgurnus inhabiting the Pad Bolshaya (Jewish Autonomous oblast), with the body length Ad 60.2-182.4 mm, body weigh 2.3-55.3 g, age 1-8 years and sex ratio about 1 : 1 (53 % males, 47 % females). Sexual dimorphism is found for antedorsal, anteventral and ventroanal distances, tail fin base length, base length and height of pectoral, pelvic and anal fins, and maximum body height. Ontogenetic variability of morphological indices in dependence on body length is detected for 26 of 32 plastic signs, whereas the dependence of anteanal, antedorsal, pectoventral, and anteventral distances, and maximum and minimum body height on the body length is not statistically significant. Males grow slower than females. Age of puberty is 4-6 years, mean individual absolute fecundity is 7409 eggs. The analysis concluded that the weather loaches Misgurnus in the Pad Bolshaya belonged to the species Misgurnus nikolskyi (Vasil’eva, 2001).

Current status of aquatic biological resources in the middle and lower parts of the Amur River basin, including Lake Khanka and the Amursky Liman, is assessed. Generally high abundance of the water organisms is noted, but a downward trend is revealed. In 2015–2019, the total annual catch in the basin by Russian fishermen changed between 15.9–69.6 . 103 t (on average 39.1 . 103 t), with the main portions of pacific salmons (31.9 . 103 t, or 81.6 % of total catch) and smelts (5.5 . 103 t, 14.1 %). After the peak in 2016, the salmons abundance in the Amur has decreased, particularly for summer chum salmon and pink salmon, the number of arctic rainbow smelt Osmerus dentex has decreased gradually in the last 3 years. The stocks of freshwater fish are generally stable, with a slight increase for some species. The program of fisheries research «Amur River Fishes» implemented for 2020–2024 includes intensifying of traditional monitoring of the stocks and their biological state, as well as organization of detailed comprehensive studies for key species. Improvement of data quality on status of the main stocks of pacific salmons, smelts, and freshwater fishes is planned, as the basis for fisheries forecasting. Besides, the program conducts quantitative assessment of the main components of the ecosystem (phyto- and zooplankton, macrozoobenthos, and fish) and their dependence on environmental changes. State of food base for the Amur fish will be evaluated, including the feeding of artificially reproduced juveniles of salmons and sturgeons. Results of these studies will allow to improve approaches to regulation of fishery and to promote development of fishery industry toward organization of effective fishery complex in the Amur River basin.

Floodplain soils function as long-lasting stock or source of different substances, including pollutants. The main factor determining biochemical processes in fluvisols is flooding. Global climate change, which is causing more frequent and massive floods, is urging us to assess the potential environmental risks and create appropriate environmental management strategies. This study was performed to estimate the impact of a heavy flood on the total content of major elements and both total and mobile trace elements in fluvisols of one of the longest rivers in the world, the Amur. The study was conducted in field conditions by sampling from the same soil profiles before and after the flood. As a result, 10 major and 42 trace elements were distinguished. Major-element composition was determined with X-ray fluorescent method, trace-element composition - with the inductive coupled plasma mass spectroscopy. Maximum decrease of concentration was determined for CaO, MnO, P2O5 (up to 60%) and Sr, Cd, Ba, Tl and Pb (up to 40%). Significant increase was in concentration of Ni, Cu, and Mo (up to 160%). Among mobile trace elements, increase was observed in concentration of Sc, Ni and Th (up to 400%). With the correlation analysis, it was also established that the main causes of changes in elemental composition of the soils were decrease of pH, development of redox environment and washing out of organic matter. The main factor determining the influence of the flood on fluvisols was floodplain relief, which affected the length of the inundation, flood water velocity and the way allochthonic matter retained.

GLDAS2.0 provides long‐term fine resolution gridded hydrometeorological data sets, which are necessary for water‐related studies, particularly in some transboundary rivers that are partially without observation. Yet, GLDAS2.0 has only been validated at limited locations, and few studies have been conducted to develop approaches to correct the GLDAS2.0 data for transboundary rivers. This work assessed the GLDAS2.0 data and developed approaches to correct their uncertainties for studies in large transboundary rivers in the Tibetan Plateau and Northeast China (NC). To achieve these goals, observational data from 1982 to 2010 and a water and energy budget‐based distributed hydrological model including biosphere after calibration and validation were employed. We find that the GLDAS2.0 data (except for wind speed) can reasonably replicate observed seasonal variations. However, its specific humidity and wind speed have large uncertainty, and precipitation has large uncertainty in summer. In NC, the trends of its precipitation, air temperature, downward longwave radiation, and wind speed are consistent with the observations. In the Yarlung Tsangpo, Lancang, and Nu Rivers, the trends of all GLDAS2.0 data reproduce the observation very well, that is, wetting, warming, and dimming trends. Validations show that the corrections are effective and the corrected forcing data can be successfully used in hydrological simulation with improved performance than the raw GLDAS2.0 data, which demonstrates the usefulness of the methodology and corrected forcing data to hydrometeorological studies in transboundary rivers in China as well as in other nearby regions/countries.

Chum salmon in the Amur River basin are represented by two races: summer and fall ones. For the summer race, one population with specific type of spawning grounds is known, but the fall race includes three populations with different types of spawning grounds. In total, four eco-geographical groupings of the species are separated in their reproduction by geographical boundaries of geomorphological zones, as well as temporally and ecologically. Chum salmon do not spawn in the Chinese part of the Amur basin in more than 50 years. In the Russian part, the summer chum salmon spawn completely in the hyporheic waters in late July — August, the major grouping of fall chum (77.1 %) spawns in the spring waters in September-November, other two groupings spawn in the hyporheic waters of the lower Amur in September (20.4 %) and in the spring waters of the lakes near the Amur mouth in October-November (2.5 %).

In the basin of the Amur River in the Russian Far East, the influence of watershed areas covered by forests on the river basin has a complex nature, and no strict functional dependency has been established yet between these two factors. A study of the Amur River watershed in the current conditions, between 2000 and 2016 (climate, forest coverage, fires, and felling), has been conducted using the ground and satellite observations. The purpose of the study was to identify their influence on the river behaviour (flow, flooding, and levels of water). The study of hydrological regime of rivers was conducted in conjunction with the analysis of the dynamics of forest and burns areas over the synchronised periods of time. A special attention was given to the changing nature of the species composition of the forests (coniferous and deciduous forests separately) from 2000 to 2016, and climatic parameters over thirty years (atmospheric temperature, dew point, precipitation). New facts have been obtained, which provide an explanation of the reasons for predominant prolonged trends in the dynamics of the summer streamflow. In the view of the general tendency toward increased forest coverage combining all species of forest stand, the trend in the dynamics of the coniferous species areas is negative. Therefore, a conclusion can be made, that one of the major factors in the increase of the river flood flow (alongside the atmospheric precipitation), is deforestation of primary coniferous forests on the watershed areas, in contrast with the deciduous forests, where the trend is positive. Practicability of such conclusions can be justified, as different types of forests have different root systems, which mellow the ground and facilitate partial loss of the atmospheric precipitation and its transformation into the groundwater flow. Besides, coniferous forests attract more frequent and intensive fires, more subjected to felling, have longer regeneration period, and also, use larger volumes of ground waters for growing and functioning. Consequently, with their disappearance, an increase in streamflow should be expected. No changes in surface temperature and humidity of the forest cover in the watersheds during 1980-2016 despite global warming. Therefore, annual variability of forested areas of watersheds is greatly influenced by fires and felling. There are reasons to assume, that because of the tendency for decreasing areas of coniferous forests, the conditions contributing to the increases in rivers&rsquo, flood flow and flood risks during monsoon and frontal cyclonic rainfalls will remain.

Dissolved iron (dFe) in boreal rivers may play an important role in primary production in high-latitude oceans. However, iron behavior in soils and dFe discharge mechanism from soil to the rivers are poorly understood. To better understand iron dynamics on the watershed scale, we observed the seasonal changes in dFe and Dissolved Organic Carbon (DOC) concentrations in the river as well as dFe concentration in soil pore waters in permafrost watershed from May to October. During snowmelt season, high dFe production (1.38&ndash, 4.70 mg L&minus, 1) was observed in surface soil pore waters. Correspondingly, riverine dFe and DOC concentrations increased to 1.10 mg L&minus, 1 and 32.3 mg L&minus, 1, and both were the highest in the year. After spring floods, riverine dFe and DOC concentrations decreased to 0.15 mg L&minus, 1 and 7.62 mg L&minus, 1, and dFe concentration in surface soil pore waters also decreased to 0.20&ndash, 1.28 mg L&minus, 1. In late July, riverine dFe and DOC concentrations increased to 0.33 mg L&minus, 1 and 23.6 mg L&minus, 1 in response to heavy rainfall. In August and September, considerable increases in dFe concentrations (2.00&ndash, 6.90 mg L&minus, 1) were observed in subsurface soil pore waters, probably because infiltrated rainwater developed reducing conditions. This dFe production was confirmed widely in permafrost wetlands in valley areas. Overall, permafrost wetlands in valley areas are hotspots of dFe production and greatly contribute to dFe and DOC discharge to rivers, especially during snowmelt and rainy seasons.

At the northwestern coast of the Okhotsk Sea, pink salmon finds its spawning grounds both in big rivers (Uda, Okhota, and some other) and small streams because the size, slopes, and structure of alluvial sediments of the majority of local water bodies are suitable for the spawning. In small streams (< 20 km long), the pink salmon prefers to spawn in the simplest parts of beds, with linear channel sloped enough for bottom infiltration, but the chum salmon usually does not spawn there. These environments are rare for large rivers of this area which are more turbulized, with the beds often transformed by floods, strongly meandering, highly branched, and sometimes splitted to several channels; however, the largest and the most numerous spawning sites of pink and chum salmons are maintained in the latter case of splitted river-bed that is typical for lower parts of the biggest rivers, though other parts of these rivers are not preferable for their spawning because of high instability of the environments. The spawning grounds of chum salmon are more resistible against floods than the pink salmon ones: they form around either parafluvial (hyporheic) or ortofluvial springs. Generally, the areas with underwater springs preferable for the salmons spawning are more usual for big rivers with variable geomorphology, numerous tributaries, and extended multichannel parts.