The Vilyui hydroelectric station No. 3 (HES-3) is being constructed as the second step of the cascade of Vilyui hydroelectric stations 141 km below the operating Vilyui HES-1 and -2 (the first step of the cascade) and 1250 km from the mouth of the Vilyui River. The closest large populated areas are: Mirnyi. the economic and administrative center of the Western Yakutia region located 80 km south of the site of the hydrostation under construction; Chernyshevskii, the village of operators of the Vilyui HES-1 and -2 and the main production facility of the cascade, located 100 km from the site along the highway: the villages Aikhal and Udachnyi, diamond-mining centers, located 450-500 km to the north, beyond the Arctic Circle. A village of the Svetlyi type was created near the construction site. The closest port and main transshipment facility is Lensk City located on the Lena River 950 krn below the port of Osetrovo (Ust'-Kut railroad station on the Baikal-Amur Mainline) and is connected with the construction site by a permanently operating highway 300 krn long. Yakutsk, the capital of the Republic of Sakha, is located 900 km east of the construction region. The main power consumers in the region are enterprises of the diamond-mining industry, composing the basis of the regional economic complex, as well as the construction industry and welfare sector. The plans of economic development of western regions of Yakutia envisage a more than doubling of power consumption by 2000. Power production of the Vilyui HES-1 and -2 has presently reached the design indices, and a further increase of the loads will lead to a shortage in the region, the only source for covering which is the Vilyui HES-3. The hydrostation with an installed capacity of 360 MW and average annual production of 1.2 billion kWh is intended to operate in the Western Yakutia power system and later in the interconnected power system of Western and Central Yakutia. The Vilyui HES-3 is being constructed in a region with a harsh, markedly continental climate, located only 2.5 ~ south of the Arctic Circle, in the permafrost zone, and belongs to regions of the Far North. The mean annual air temperature is 9.7~ the mean January temperature is minus 35~ the observed extreme values of the January and July temperatures are respectively minus 63 and plus 36~ The average duration of the snow cover is 220 days, freezeup 190-220 days, and heating period 270 days. The average annual amount of precipitation is 330 ram. The drainage area of the Vilyui at the site of the hydro development is 156,000 km 2, the average annual discharge is 681 m3/sec. The natural hydrologic regime of the river has been altered to a considerable extent due to carryover regulation by the reservoir of the Vilyui HES-1 and -2. The Vilyui River at the site flows in a narrow canyon-like valley with slopes with a steepness up to 30-45 o without expressed terraces. The channel width at the waterline at the site of the hydrostation is about 150 m, the valley width at the elevation of the normal pool level (NPL) is 350-500 m. The divides and upper parts of the valley slopes are composed of dolerites, in the lower part of the slopes and channel part of the valley occurs a carbonate-argillaceous intercalating stratum of marls, limestones, dolomites, and sandstones severely fractured and here and there weathered to loams. The river slopes and bottom are covered by a stratum of Quaternary deposits. The permafrost with a thickness up to 300-320 m and temperature to minus 9-11~ is interrupted by talik under the river channel. The ice content of the perennially frozen bedrocks is 5-10 %, of the floodplain and slope deposits 5-25 %, and in individual stretches reaches 40-60%. The pores and fractures of the lower level of permafrost are filled with saline waters with a negative temperature (cryopags). Landslide formations in the form of blocks, lumps, rubble, and grus are developed on the left-bank valley slopes.