Filling of the reservoir of the Ust'-llim hydroelectric station, located 303 km downstream of the Bratsk hydroelectric station, began in October, 1974. In December, 1974 the water level in the reservoir reached the start-up mark, and by the end of the year the first three units of the hydrostation were put into operation. By the end of 1977 all 15 units of the first phase were started up [i]. In 1978-1979 the last (16th) unit was put into operation, and in December, 1980 the Ust'-Ilim hydrostation was received into industrial production.The ice and thermal regimes of a river change substantially with the construction of hydrostations.This is due to a change in the discharges and temperature of the water in the lower pool. To study the character of the change in the ice and thermal regime of the pools of the Ust'-Ilim hydrostation and its effect on the region of construction of the fourth reach of the Angara cascade of hydrostations --the Boguchany --being constructed 386 km downstream of the Ust'-llim, the following observations are being conducted in the investigated region: in the upper pool --thermal observations of increased accuracy on a permanent vertical located 1600 m upstream of the dam and 1800 m from the left bank; transverse thermal sections along a section line located 1600 m upstream of the dam and passing through the permanent thermal vertical; and longitudinal thermal sectionsat a distance up to 40 km from the hydrostation; in the lower pool --measurements of the water temperature in the tailrace directly at the dam opposite each even unit with subsequent averaging of the data obtained; transverse thermal sections on thermal section lines located at a distance of 1.5, 4.5, Ii.0, and 15.5 km downstream of the hydrostation; longitudinal thermal sections at a distance of 40 km from the hydrostation; and observations of the position of the ice edge, water levels and discharges, and ice situation. The water temperature in the pools is measured with an accuracy to 0.OI~ The middle course of the Angara, where the Ust'-llimhydrostation is constructed, is located in a region with a markedly continental climate. The mean annual temperature at the nearest weather station, Nevon, is --3.9 ~ The normal temperature during the period October-May is --12.5 ~ .During the years of operation of the hydrostation from 1974 to 1980 the coldest fallwinter period was observed in 1976-1977 and the warmest in 1977-1978. Westerlies prevail in the region of the pools of the Ust'-llimhydrostation.The maximum wind speeds are observed mainly in the fall months, when the temperature regime of the reservoir, which to a considerable extent determines the variation of the water temperature under the ice in the upper pool and, consequently, the temperature of the water entering the lower pool, forms.After filling to the normal pool level (NPL) in May, 1976 the Ust'-llim reservoir represented a water body 303 km long and 90-92 m deep in the near-dampart.The annual thermal cycle of the reservoir includes thermal periods characteristic...

The construction of hydroelectric plants modifies the natural ice regimen of rivers since, in addiuon to the usual physicogeographical factors, the ice conditions are determined by the flow and temperature of the water leaving the reservoir. As a rule, under flow-regulation conditions the winter flows are increased over the natural regime. Because of this, the passage of increased flows under the ice cover, which is attended by an increase in channel resistance, causes a substantial rise in the water level. Of still greater significance is the magnitude of the outflows during the winter and spring periods, when the ice cover is formed on the river, as well as during the process of opening of the river (after breakup of the ice). Of special interest in this connection is an examination of the ice conditions observed at the downstream side of the Bratsk hydroelectric plant, from the viewpoint of regulation of the ice regimen in the region oft~enext downstream plant in the system--the Ust'-Ilim hydroelectric plant.Ordinarily, during the different construction stages the river channel is substantially constricted by the structures being constructed, and the passage of large ice floes becomes a serious problem [1]. Thus, at several hydroelectric plants now constructed the ratio of the ~ddth of the ice-passing front to the ~,-idth of the river upstream from the spillway crest during construction was as follows: at the Ust'-Kamenogorsk plant, 0.20; at the Bukhtarma plant, 0.28; at the Novosibirsk plant, 0.18; at the 8ratsk plant, 0.16; at the Mamakan plant,0.5-0.08; at the Krasnoyarsk plant 0.19-0.077; and at the Ust'-Ilim plant, 0.11.A substantial reduction in the width of the ice-passing front over the natural width affects passage of the ice through the structures under construction and leads to an increase in the probability of formation of large ice obstructions and accumulations.The hydraulic conditions and indices of passage of ice through the crest of the Ust'-Ilim hydroelectric plant are presented in Table 1.Under operating conditions during the ~,inter, water at a positive temperature passes from the reservoir to the downstream side of the Bratsk hydroelectric plant. During the initial operation period the water temperature dropped to 1.02*C at the beginning of April. Subsequently, the mean 10-day water temperatures in the Angara River at the downstream side of the Bratsk plant were in the range 1.65-4.58"C from November to May. Such water temperatures determine the maintenarice of a polynia (unfrozen patch of water in the ice) during the winter, the dimensions of which vary according to the meteorological and hydraulic conditions. As is known, ice formation in the downstream polynia takes place below the zero-temperature profile. The analytical positions of maximum approach of the zero-temperature profile to the dam at the downstream side of the Bratsk hydroelectric plant, obtained by the heat balance method [2], are presented in Fig. 1. It should be noted that the values of the air temperature presented in T...

Filling of the reservoir for the Ust-llim hydroelectric scheme in conjunction with the two upstream hydropower developments --Irkutsk and Bratsk --under the rigorous climatic conditions of Eastern Siberia, if of great interest from the hydrothermic viewpoint because the hydrothermal regime of the reservoir plays a determinant role in the dynamics of the iceforming processes downstream.The value of an investigation of the hydrothermics upstream and downstream of the Ust-llim hydroelectric scheme is increased still further if one bears in mind that, some 380 km below the Ust-llim site, construction has commenced on the Boguchany hydroelectric scheme --the next step in the Angara cascade.Filling of the Ust-llim reservoir was begun on Oct. 16, 1974. The rate of water-level rise was 60-70 cm/day in November and 40-50 cm/day in December.The first two turbines of the Ust-llim hydroelectric station were placed under load at the end of December, when the upstream level had reached an elevation of 76.56 m, the volume stored was 19.65 km 3, and the water-surface area was 930.9 km 2. The river flows were passed through the two turbines and six submerged outlets.Presented in Table i are data which indicate the rate of reservoir filling and the depths in the part adjoining the dam on the first day of each month.The full-scale investlgations of the hydrothermalconditions above and below the Ustllim hydroelectric scheme during reservoir filling were conducted by the Angara expedition of the S. Ya. ZhukAll-Union Scientific-Research and Deslgn-Investigation Institute of Hydrotechnical Construction (Gidroproekt).The investigations included observations, to a raised standard of accuracy, of the water temperature along fixed vertical lines; measurements of water temperatures along transverse and longitudinal thermal sections in the portion of the reservoir adjoining the dam; and also downstream at two fixed verticals and four transverse thermal sections.The fixed thermal vertical upstream was located 2000 m above the dam site and 1300 m from the left-hand bank.The thermal site upstream passed through the fixed thermal vertical, and the longitudinal thermal sections had a length of 39 ~m upstream from the dam. The fixed thermal verticals downstream were located in the tailrace, 80 m from the hydroelectric station, and below the exit from the submerged outlets at a distance of 400 m from the dam. Thermal sites downstream were located 4.5, ii.0, and 15.5 km from the hydroelectric station. Measurements at all the thermal verticals and sites were made with an accuracy of 0.01~The meteorological conditions in the region of the investigations during 1974-1976 can be evaluated from the data recorded at the Nevon meteorological station (Table 2). It is evident that the mean annual air temperature in 1974 was 1.5~ less than the long-term mean and that the mean monthly temperatures for October and November were 5.2 and 3.5~ below normal. TABLE i. Rate of Reservoir Filling, m 1974 1975 1976 Mon~ levels I deptl~ levels depths I II Ill IV V VI VlI...