This chapter contains a comprehensive description of the Volga River, the largest river in Europe, and its main tributaries, the Kama, Oka, Sheksna. It starts with a review of human history and biogeographical setting including paleogeography of the basin, as well as description of physiography and climate, and features of hydrological and biogeochemical regime in the past and present. Problems of the management, conservation, and economic importance of the basin are also under consideration. Much attention is paid to aquatic and riparian biodiversity. These sections include description of the species composition of macrophytes, algae, aquatic invertebrates, and fish with an assessment of the abundance, seasonal, and long-term dynamics of biological communities. Data on invasive species from Ponto-Caspian and Boreal-Arctic complex are given. Molecular studies performed during the last 10 years significantly clarified fauna composition as well as phylogeographic patterns.

Long-term dynamics of water level in Lake Nero (1930–2011) has been analyzed. Pronounced trends, determined by the climate and anthropogenic components have been identified. Up to the 1970s, at a drop in lake water level, the ecosystem still showed the main features of a shallow water body overgrown with macrophytes. Water level rise caused a more intense development of phytoplankton, an increase in biogenic element concentrations, a decrease in water transparency and the contribution of cyanobacteria of S 1 codon; pondweed disappeared almost completely. Step-by-step regression analysis of phytoplankton development indices and hydrological characteristics showed that water transparency in Lake Nero depends by 80% directly on water exchange and inversely on the concentration of chlorophyll a in seston. The main hypothesis of the study was confirmed: the key trigger in phytoplankton–macrophytes competition is water level rise.

Trends in changes in the total solar radiation and water temperature in the Rybinsk reservoir have been analyzed on the basis of the long-term observations (1954–2013) carried out by the Rybinsk Hydrometeorological Observatory. Direct correlation was established between water temperature and solar radiation over the vegetation period and indirect correlation – between radiation and amount of precipitation on the water area of the reservoir. It was shown that water temperature is affected not only by solar radiation and air temperature, but by the water content of the year as well.

Presented are the results of studying the water temperature changes in the Rybinsk Reservoir during the ice-free period caused by the climate warming. Linear trends are revealed and estimated. The trend is observed for the period of 1976–2008 towards the increase in the average water surface temperature during all months at the maximum rise rate of 0.89°C/10 years in July. It is demonstrated that the average water temperature in the reservoir in May–October has been above the norm since 1995.

Peculiarities of formation and seasonal dynamics of hydrological structure of the Sheksna River Deep of the Rybinsk Reservoir are considered. The contribution of various genetic water types, including run-offs of the town of Cherepovets, to the formation of its water masses is established on the grounds of model calculations. It is shown, that the hydrological structure effects the distribution of hydrobionts in sewage disposal areas. The assessment of the contribution of Cherepovets run-offs to the change of total water salinity of the deep is given.

Variations of hydrometeorological variables of the Rybinsk Reservoir area from 1947 to 2005 are analyzed. A special attention is given to the global warming period started since 1976. It is shown that the intensity of air temperature increase on the reservoir shore during recent 30 years made up 0.46–0.56°C/10 years. The maximum increase in the water temperature at shore stations and in the surface layer was registered in July at an increase rate of 0.7–1.2°C/10 years. The change in climate conditions resulted in the increase in low-water runoff, decrease in snowmelt flood volumes, and shift in the time of snowmelt flood start.

The Volga River, at 3690 km, is the longest river in Europe and 16th in the world. The geographical situation of the Volga promoted human colonization by various nations and played an important role in the movement of people between east and west (from Asia to Europe) as well as south and north. The present ethnographic composition of the Volga region consists of Indo-European (Russian majority, Ukrainians, Germans, Belorussians, Poles, Latvians), Finnish (Votyaks, Permyaks, Chemerisis, Mordvas, Karels) and Turkish (Bashkirs, Kyrgyzs, Tatars, Chuvashs) peoples. Kalmyks, representatives of Mongol nationality, form a separate group. The Volga basin is divided into three sections referred to as the upper, middle, and lower Volga. Gorky dam and Kuibyshev dam are considered the border of the upper and middle Volga and the middle and lower Volga, respectively. This chapter includes subsections describing each region of the Volga, along with separate sections on its largest tributaries: Kama, Oka and Sheksna.

Results are presented of the mineralization and salt composition changes of the Rybinsk reservoir and its tributaries’ water over a period 1950−2002. Role of the anthropogenic component in the ion yield of the Volga River (station Myshkin) for some periods has been assessed.

On the basis of the data obtained during field observations in the summer low water period of 2001, the patterns of chlorophyll distribution and its relation to hydrological and hydrochemical factors in two eutrophic reservoirs of the Middle Volga are studied. The hydrological structure of the Gorky Reservoir, where the Volga water mass dominates, is homogeneous, while in the Cheboksary Reservoir along with the eutrophic Volga waters, the mesotrophic Oka water masses can be distinguished keeping their abiotic and biotic features over a long distance. Phytoplankton in the two contiguous reservoirs with different flow regimens and anthropogenic loads responds differently to the external influence. An autotrophic community in the Gorky Reservoir is more stable and depends little on abiotic factors which account for ∼63% of the explained chlorophyll variation. In the Cheboksary Reservoir under maximal for the Volga cascade flow velocity and anthropogenic load, the development and distribution of phytoplankton are almost completely ( R 2 = 0.93) controlled by these factors. The trophic state of the reservoirs has not changed as compared to the beginning of the 1990s.

The results of comparative studies of fluorescent characteristic of humic substance and electroconductivity in the surface water layer in the Rybinsk Reservoir are presented. The hydrological structures obtained with the use of these characteristics generally coincide. This opens the way to the use of humic substance fluorescence, which allows remote measurement, as an indicator of the hydrological structure.

Seasonal and long-term variability of the Rybinsk Reservoir level regime over the period of its operation was analyzed. Changes in the level regime at different phases of water content were shown together with the data on mean monthly and extreme level values and the rates of the reservoir filling and drawdown. The level regime impact on the functioning of individual components of the reservoir ecosystem was discussed. Due consideration was given to the assessment of changes in the reservoir environmental situation under the condition of the normal impounded water level drop by 1 m.

The characteristics of zooplankton of the Sheksna Reservoir are presented for years with different water abundance and weather conditions. Zooplankton of Lake Beloe (Belozerskii Pool of the Sheksna Reservoir) is shown to retain the characteristics of a limnetic community, which was typical of the lake before the onset of Sheksna runoff regulation, with a predominance of crustacean biomass.

In the paper it is proposed to use fluorescence of dissolved organic matter (DOM) as a water masses indicator. The DOM fluorescence spectra were registered and for two reservoirs of different types: the costal zone of the North Sea and the Rybinskoe reservoir in the middle of the European part of Russia. In the paper it is shown that using normalized intensity of DOM fluorescence and artificial neural networks (ANN) for analysing DOM fluorescence band shape can provide determining the water masses type in natural reservoirs.

Relationships between the principal characteristics of benthic communities, physical characteristics of soil, depth, current rate, and the concentration of pollutants in the soil and water of the Gorky and Cheboksary reservoirs are investigated. Numerous significant correlations are found to exist between these characteristics. It is recommended that the assessment of water quality by characteristics of benthic communities should be made with a great caution.

The social and economic role of the Rybinsk Reservoir is assessed, and its hydrological, hydrophysical, and hydrochemical features are characterized. The reservoir biota formation is described and its potential for fishery is determined. The attitude of the state structures and the community toward the reservoir in different periods of its existence is discussed.

The results of studies of the Ivankovo, Uglich, Rybinsk, and Gorkii reservoirs are analyzed. The studies involved a wide range of hydrochemical (salt composition, biogenic elements, organic matter, oil products, and heavy metals) and hydrophysical characteristics in the summer low-water period. The concentrations of organic matter and major biogenic elements are shown to decrease downstream along the chain of reservoirs with a respective general decrease in the trophic status. Large built-up areas located downstream of the Rybinsk Hydroelectric Plant are shown to adversely affect the river water quality.

Long-term changes in chlorophyll (CHL) during 1969 to 1984, in connection with hydrological conditions, were studied in two basins of the Rybinsk Reservoir (the third step of the Volga river system, northwest Russia). Year-to-year CHL fluctuations are quite similar in the Volga Reach and Main Part of the reservoir and depend upon hydrometeorological factors. Increases in the mean seasonal, most frequent and summer maximum CHL concentrations, demonstrate the accelerated eutrophication of the Main Part of the reservoir, which holds most of the total storage. Effects of water content, characterised in terms of total input, water level and water exchange intensity, are the most significant among hydrological factors. A clustering of years in fields of the first and second principal components corresponds to high-water periods (1969-1975) and low-water periods (since 1976). Strong influences of water content - controlled by human activity - on phytoplankton abundance (CHL) make it possible to provide ecosystem monitoring and management. Consideration is also given to the role of hydrological characteristics, including water balance, in producing undesirable changes that have occurred during recent yeats.