Based on the data of complex environmental studies, which have been regularly carried out at six standard stations of the Rybinsk Reservoir since the middle of the 20th century, the orientation of changes in the elements of the reservoir ecosystem associated with global climatic events has been analyzed. During the period of climate warming, which began in 1977 and continues into the 21st century, the air temperature in the warm season increased by 0.9°C, the water temperature by 1.4°C, the average annual inflow by 7.5%, and the duration of the ice-free period by two weeks. An increase in electrical conductivity and color of water, a decrease in transparency were noted. With significant interannual variations in biological characteristics, in the XXI century, the number of bacterioplankton increased by 1.7 times, and bacterial production doubled. Chlorophyll content increased by 1.4 times and values 15 µg/L reflecting the eutrophic state of the reservoir began to be observed more often. In the biomass of phytoplankton, the proportion of small cell forms increased. The total abundance of phytoplankton increased due to the development of cyanobacteria, which form a long summer maximum in the seasonal dynamics of the community. The increase in water mineralization promoted the progressive spread of alien brackish-water algae. Biomass of zooplankton increased by 2.5 times. An increase in the abundance of crustaceans (Cladocerans, by 1.6 times; Copepods, by 1.9 times) caused a change in the structure of zooplankton and the formation of a strong late summer peak of biomass. The intensification of hydrobiological processes was clearly manifested after the abnormally hot 2010, the conditions of which not only stimulated the development of plankton communities, but also but also caused the formation of oxygen deficiency in the bottom layers. Warming has significantly transformed the ecosystem of the Rybinsk Reservoir, intensified eutrophication processes and worsened water quality. Changes in hydrometeorological characteristics have gone beyond the mild scenario of climate warming.

The study is focused on variations of air temperature in the basin of the Volga reservoirs, the total inflow into the water bodies, their water exchange, water level and temperature, and the heat content of water mass in open-water period under various climate conditions. The object of the analysis is the long-term series of hydrometeorological data processed by statistical methods. It is shown that the present-day air temperature has increased by 1.3‒1.8°C compared with the period before 1976. The rate of warming was on the average 0.50°C/10 years. The volume of annual inflow increased by 12.4%. Three low-water and 4 high-water phases were identified in the reservoirs of the Upper Volga, including 29–31 low-water, 25–31 high-water, and 8–16 medium-water years. During the low-water phases, the volume of inflow into the reservoirs is 10–28% less than the long-term average, while in high-water years, it is 4–20% higher. The coefficient of water exchange in the reservoirs decreased or increased by 5–13% relative to the values obtained earlier. An increase in the winter and a decrease in the spring inflow were recorded in the reservoirs of the Upper Volga and in the Kuibyshev Reservoir. A tendency toward an increase in the normal annual water level was observed in the reservoirs in the upper part of the Volga and in the Kuibyshev Reservoir, while in the lower Volga, the normal annual level somewhat decreased. In low-water phases, the reservoir levels were on the average 17 cm below and in the high-water phases, 10 cm above the normal annual value. An increase in air temperature during the warm season in the reservoir water areas, on the average by 1.2°C, led to a synchronous increase in the temperature of the water mass by 1.1°C. At the same time, the heat content of the water mass of the reservoirs increased, on the average, by 24% in the upper part of the Volga and by as little as 2–11% in its lower part.

Long-term features of the vertical and horizontal structure of water temperature field in Lake Sevan are discussed. It is shown that the climate warming has led to an increase in epilimnion temperature in Bol’shoi Sevan in July by 2.0‒3.0°C. The increase in water temperature in hypolimnion was not greater than 1.1°C. In autumn (October), the epilimnion became 1.2°C warmer, while hypolimnion temperature practically has not changed on the average over years. Temperature fields were used to calculate the density currents in summer and autumn periods. A dominating cyclonic water circulation was revealed all over the lake, confirmed by chlorophyll distribution by satellite image data. In the case of large horizontal gradients of water density, the flow velocity can reach 50 cm/s. Autonomous buoy stations revealed a wide range of water temperature variations due to internal waves of different nature. The reversible vertical mixing of water mass by internal waves plays an important role in the distribution of nutrients and plankton within the water mass. The water level rise by ~3 m, unlike it drop by 1981 by 18.48, has not caused any significant changes in lake hydrological regime.

Changes in the higher aquatic vegetation (HAV) of the protected shallow water “Krasnyi Ruchei” from 1989 to 2019 have been analyzed based on aerial imageries, multidate satellite images, and direct field studies. Images from the second half of July to late September are used to study of the overgrowing. An inverse relationship has been found between the degree of overgrowing and water level. The degree of overgrowing of the shallow water is highest (82–86%) at low values of the water level (99.6–99.7 m) and slightly lower (63–78.8%) at higher values (100.3–100.7 m). This is inconsistent with the data on the Rybinsk Reservoir published before 2013; therefore, further research based on a more representative material is required. The specificity of the regulation of the Rybinsk Reservoir water level, as well as the predominant shallow water phase during the freeze-up period and throughout most of the vegetation season and high sedimentation rates, create favorable conditions for the distribution of HAV. By 2019, HAV in the shallow water had spread to a depth of 1.8 m and the area of overgrowing reached 73% of the normal water level. Helophytes occupied about 70% of the optimal shallow water area, while hydrophytes only 20%. Taking into account the high rate of sedimentation and shallowing, the rates of overgrowing of the studied shallow water will be more rapid in the nearest future, which can be controlled by maintaining a water level close to the normal water-surface elevation level of 101.81 m.

The morphometric characteristics and bottom topography of typical shallows behind the island are characterized. The long – term and seasonal dynamics of the water level in the Rybinsk reservoir during the period from 1947 to 2020 is considered. The periods of fluctuations of the main hydrometeorological parameters in relation to the investigated section of the reservoir are shown. Based on archival and modern observation data on the structure of currents, the patterns of water transport and water exchange in the shallows are investigated, taking its seasonal overgrowth with macrophytes into account.

This study considers the possibility of using geometric models in the study of individual elements of hydrological regime in different types of protected shallows in the Upper Volga reservoirs. Three types of models were chosen and verified. The formulas for calculating the bathygraphic curves of geometric figures approximating the beds of protected shallows are determined. Classes with different morphometric characteristics are identified using non-hierarchical cluster analysis for each type of shallows. Model calculations of areas and volumes of shallows are carried out depending on seasonal changes in water level, as well as estimates of thermal content, average monthly water balance and water exchange in the shallows.

The long-term and seasonal dynamics of the water level in the Rybinsk Reservoir has been studied for the period of 1947–2015. The dependence of the area of overgrowing of shallow waters (S) on the water-level fluctuations (Z) has been obtained: S = 30‒45e1.054Z. The overgrowing of open shallow waters is restrained by the hydrodynamic influence on the littoral zone, while, in the case of protected shallow waters, the limiting factor is the drying up of habitats of aquatic plants, which takes place at the end of the vegetation season. For different ecological groups, the calculated maximum and optimum areas of overgrowing vary from 10 to 100% of the total available shallow water area. Reserve areas suitable for the possible future distribution of plants vary from 40 to >90%. Results of the study show that, due to sharp long-term and annual fluctuations in water level, actual areas occupied by a higher aquatic vegetation are always significantly less than the calculated optimum and maximum areas. Under the current regime of reservoir regulation, its overgrowing will be maintained at a current level for a long time.

The features of overgrowing of different types of protected shallow waters in the Ivankovo, Uglich, and Gorky reservoirs have been considered. The average degree of overgrowing increases in the following series: pocket-type shallow waters (37.3% of area), shallow waters behind islands (40.6), and bays (56.9). Helophytes contribute to the overgrowing of more than 50% of all types of protected shallow waters. The composition of plant community dominants of protected shallow waters of different types is very similar, because the reservoirs belong to the same cascade, are located in one climatic zone, and have similar ecotopes. The effect of morphometric parameters (area, volume, length, average width and depth) and their ratios (oblongness of water area, openness, and isolation) on the overgrowing of protected shallow water areas is expressed only at average values of these parameters and has an exponential form. The contribution of the area isolation to overgrowing is most significant (up to 55%).

Przedstawiono cechy ogolnej cyrkulacji wod w zbiorniku wloclawskim oraz zroznicowanie parametrow falowania w roznych sytuacjach hydrologicznometeorologicznych. Wyznaczono obszary dna zbiornika podlegające rozmywaniu pod wplywem dynamiki wody. Obliczenia symulacyjne cyrkulacji wod wykonano przy uzyciu oprogramowania stosowanego w Instytucie Biologii Wod Środlądowych Rosyjskiej Akademii Nauk w badaniach hydrodynamiki zbiornikow zaporowych na Woldze. Wyniki symulacji rozkladu prądow wodnych wykazaly, ze niezaleznie od warunkow wiatrowych w rzeczno-jeziornej cześci zbiornika cyrkulacja wody ma charakter typowo rzeczny z wyraźnie znaczonym prądem przeplywowym. W szerszej jeziornej cześci zbiornika, w przypadku wiatru z sektora zachodniego, tj. z kierunku przeciwnego do kierunku przeplywu rzecznego, w lewobrzeznej, pozakorytowej, cześcizbiornika formują sie lokalne komorki cyrkulacyjne. Powstają one w wyniku interakcji zachodzących miedzy prądami przeplywowymi a wiatrowymi. Prawdopodobienstwo uformowania sie takiej cyrkulacji prądowej jest duze juz przy predkościach wiatru 1,5-2,0 m·s-1i malych, do 500 m3·s-1, doplywach wody do zbiornika. Wyniki modelowania wykazaly, ze dla wiatru wiejącego wzdluz osi zbiornika z predkością 8 m∙s-1, w jego centralnej cześci, gdzie dlugości rozbiegu fal są wieksze od 3 km, formują sie fale o wysokościach dochodzących do 50-70 cm. W innych cześciach zbiornika wysokośc falnie przekraczaly 20-30 cm. Na podstawie obliczonych krytycznych naprezen stycznych w naddennych cześciach fal i prądow wodnych określono, ze dominujący wplyw na erozje dna w plytkowodnych cześciach zbiornika mają prądy przeplywowe oraz wiatrowe. Efektem oddzialywania prądow wodnych na dno zbiornika są strefy erozyjne ciągnące sie wzdluz calego lewego brzegu zbiornika oraz lokalnie wzdluz brzegu prawego. Wyniki przeprowadzonych badan wykazaly, ze zastosowane symulacje numeryczne mogą byc z powodzeniem stosowane nie tylko w badaniach hydrodynamiki plytkich jezior i zbiornikow zaporowych typu jeziornego, ale rowniez w symulacjach dynamiki wody w plytkich cześciach silnie przeplywowych zbiornikow zaporowych typu dolinnego.

In 2008, transition (gradient) and stable zones in backwaters of the tributary of the Rybinsk Reservoir were distinguished according to water conductivity. The highest values of BOD5 (biochemical oxygen demand) and the maximum zooplankton abundance within the backwater zone as well as in comparison with the neighboring systems (the river and the reservoir) were recorded in the transition zone where the marginal effect appeared.

The direction and rate of sedimentation processes in variable-backwater zones—upstream river-type reaches (lower pools of hydropower systems) and large bays of reservoirs of the Volga chain (many-year aspect, mesoscale level) and in the deltas of median and small rivers in the Upper Volga region, regulated by beaver ponds (seasonal aspect, microscale level) are characterized. The changes in bed relief and the structure of sediments in the examined zones were found to have common features and pass all stage of bed and shore formation typical of lowland reservoirs-from active erosion-abrasion activity to its gradual decline and from intense to passive sedimentation.