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...