Characteristics of the regime and structure of water masses in the Volga reservoirs N. V. Butorin Abstract. The characteristics of the hydrological regime of reservoirs is shown in peculiarities of the water balance, water circulation, level, wave, temperature and ice regimes. The peculiarities of the hydrological regime lead to heterogeneity in the physical and chemical characteristics of the water. Detailed analysis of these characteristics makes it possible to identify distinct water masses with different properties in reservoirs. Resume. Los traits spécifiques du regime hydrologique des reservoirs se manifestent dans les particularités du bilan hydrologique, de la circulation de l'eau et dans les particularités des régimes des vagues, des températures et des glaces. Ces particularités du régime hydrologique conduisent à l'hétérogénéité physique et chimique de la masse d'eau. L'analyse complexe des champs de ces caractéristiques nous permet d'établir la présence dans les réservoirs des masses d'eau présentant des particularités différentes. The use of natural water resources under modem technological progress is limited by the peculiarities of their geographical distribution, their seasonal and yearly fluctuations and also by considerable water pollution from domestic and industrial waste. One of the most effective means of controlling water resources is by creating reservoirs. Reservoirs are becoming an essential feature of the landscape. Each of them is a complex geographical object. Reservoirs differ from other water bodies in their bed morphology, in the interaction of the water mass with the bottom and the shores, in the physical and chemical properties of the waters and their dynamics, The main characteristic feature of a reservoir is its slow water exchange and the possibility of regulating it. The latter results in peculiarities primarily of the water balance and also in the water regime of reservoirs. On the input side of the balance, supply to the reservoir comes from inflowing rivers. On the output side of the balance, the entire water discharge is through hydro technical structures. Therefore, the peculiarities of the reservoir water balance are primarily conditioned by the economic considerations. The characteristics of a reservoir's water balance allows it to be considered as a special type of water body and this determines the peculiarities of the hydrological regime. The peculiarities are clearly shown in changes of water level. Many of the Volga reservoirs are characterized by relatively constant level in the summer—autumn period and by rather low levels in winter. In reservoirs of the lake type, wind causes considerable fluctuations of the water level. This type of level regime is associated with reservoirs exhibiting great changes in the surface area. Thus, in the Rybinsk Reservoir in some years, the area of the water surface in winter may be only be half as large as in summer. Considerable change in the area of reservoirs greatly affects the biological processes. Flooding and drying up of the littoral zone, and fluctuations of depths change environmental conditions for aquatic animals and plants. The complexity of the wave regimes found in reservoirs results from their individual characteristics of area, depth and shore configuration. These impose limitations on the size of maximum wave formation and resultant agitation. In reservoirs built on the extensive plains of the Soviet Union the wave processes are greatly developed. Characteristics of water masses in the Volga reservoirs 493 They considerably Influence the water dynamics, bottom formation, shore transformation and exploitation of reservoirs. The regulation of the reservoir and the natural morphology produces a complex system of currents. In reservoirs of the river type discharge currents play a decisive role in the transfer and circulation of waters. In water bodies of the lake type these kinds of currents are only seen in the valleys of the area where backwater is displaced, whereas in the more open areas the circulation is due to the wind. In association with the change in the current regime in the reservoirs great changes have occurred in the regime of solids discharge and, consequently, in the values of turbidity and transparency of water. The most significant decrease in turbidity and increase in transparency are observed in backwater zones and in vast lake-like parts of reservoirs with a slow flow. The heating of different parts and water layers of reservoirs takes place unevenly. There is a vertical thermal stratification. The distribution of temperature in bottom sediments is quite peculiar. In the lake-like parts of reservoirs, the temperature of the bottom, and near-bottom, layers of water continuously rises under ice. This leads to some increase in the total heat storage by the end of winter. In the river-like parts of reservoirs the opposite process applies. The peculiarities of the hydrological regime of reservoirs lead to heterogeneity of physical and chemical characteristics of the water. Detailed analysis of these characteristics has made it possible to identify different water masses in reservoirs each with different properties. The term water mass is defined as a relatively large volume of water formed under certain geographical conditions of the basin or in the water body itself, having nearly constant values during each phase of the hydrological regime. It also has a relatively even distribution of physical, chemical and biological characteristics and constitutes a united complex and as it spreads it retains its individuality. The main task in the analysis of water mass for any water body is to distinguish the separate identities of the different water masses and to characterize diem qualitatively. This can only be done if objective and physically-based criteria are selected. In the Volga reservoirs the selected criteria are temperature, eleciro- conductivity, transparency, colour and contents of certain ions. In the Rybinsk Reservoir the heterogeneity of the water is revealed by breaking down the physical and chemical characteristics into natural components. The fields of the first and the second expansion vectors of electroconductivity, hydrocarbonates, hardness and calcium show that the waters of the central part of the reservoir and the parts near the mouths of the main rivers have not only some physical and chemical peculiarities but actually have quite a different character depending on the index chosen. Reservoirs are intermediates between rivers and large lakes in the chain of runoff processes and water exchange. The peculiarities of water mass formation in them are as follows. In reservoirs of seasonal regulation the spring river water displaces the winter water completely. When the spring flood is over such reservoirs are filled with the river water masses which has been modified due to mixing. In the deeper parts of the water bodies a thermal stratification arises, deposition of suspended river sediments takes place, the phytoplankton develop vigorously and the gaseous regime and optical characteristics change. These processes transform the river waters to such an extent that a lake water mass typical of the reservoir itself is formed. The lake water mass is best seen in summer. In autumn and especially in winter it is displaced by modified river water. In long-period storage reservoirs, the lake water mass formation takes place due to the dilution of the winter water by the spring flood river water. In the winter water there is a nucleus representing the main volume with fairly uniform properties. The 494 N. V. Butorin zone of contact of the lake water mass with the river water, having the greatest gradient values of the characteristics, is the hydrological front. The lake water masses exist in such reservoirs during the whole year and in volume dominate the river water masses (Table 1). TABLE ]. Pcrtenlape ratio of volumes, of water masses in the Rybinsk Reservoir Volga Mologa Sheksna Characteri;ilic s of rescrvoii• on water wa 1er water Reservoir date of observation mass mass mass itself Date of Level Area Volume Valu me Volume Volume Volume 2 observation (m) (km ) (km') (%) m (%) (%) I960 9 Mit)' 100.17 3762 18.68 10.3 14.4 11.3 64.0 3 June 100.50 3910 19.94 11.2 16.6 18.0 54.2 28 July 99.85 3618 17.45 6.8 2.3 8.6 81.8 4 October 99.65 3528 16.76 4.1 2.9 8.8 84.2 27 October 99.47 3447 16.15 4.2 3.0 11.0 81.8 1961 14-24 l-'eb. 99.65 3528 16.76 10.0 14.4 8.2 67.4 17-24 March 99.45 3438 16.08 10.2 13.4 11.0 65.4 12 May 101.87 4593 25.71 13-6 11.8 11.4 63.2 8 June 102.11 4715 26.87 18.4 12-7 16.2 52.7 3 August 101.80 4545 25.38 3.4 2.6 3.7 90.3 10 October 101.28 4585 23.14 3.5 2.5 7.0 87.0 13-15 Nov. 100.83 4060 21.21 3.5 3.4 - - 25-28 Dec. 100.05 3708 18.22 9-0 3.4 - — Investigation of the water mass structure has shown that even in a shallow reservoir such as the Rybinsk Reservoir there exists a distinct vertical heterogeneity in some seasons. Evaluation of the percentage contents of water masses with the help of nomograms of the hydrological characteristics (Butorin, 1969) has made it possible to show the peculiarities of the vertical structure of the waters in this water body (Table 2). The vertical heterogeneity of the water in reservoirs is caused by the difference in temperature and sali concentration in the river waters and the reservoir. The temperature variations in the river waters feeding the reservoir, and in the water accumulated in it, are not synchronous in the yearly cycle.