The influence of water level fluctuations in the Ust-Ilimsk water reservoir in Russia on the action of some geological processes

The Ust-Ilimsk water reservoir is located in the taiga zone in the Central-Siberian Upland. It was created between 1975-1977 as a result of storage of flood waters and water from the higher located Bratsk Reservoir. With an area of 1873 km2 and a water capacity amounting to 59.4 km3 it belongs to the group of largest water reservoirs in the world. It is the third level of the Angara cascade, projected as a multifunctional object - for the efficient functioning of the Ust-Ilimsk Hydroelectric Power Station, to streamline navigation on the lower Angara, to fulfil municipal and industrial water needs and also for floating timber. On the basis of the author’s own investigations and source materials obtained from the Irkutskgidromet and the Institute of the Earth’s Crust in Irkutsk, the fluctuations of the water level and the volumes of water outflow in the period 1975-2007 and the volume of processed material at the selected water shores in the period 1977-1990 were analysed. In addition, the data of seismic events in the region of the Ust-Ilimsk water reservoir were compiled. During this period the functioning changes of the reservoir in terms of the capacity of the water mass occurred under the influence of both natural and anthropogenic factors. It was determined that the creation of this natural-technical water object activated two geological processes in its shore zone - abrasion and induced seismicity. It was stated that the especially large transformations of the reservoir shore zone by these dangerous processes occurred in the first years of its intensive exploitation, in the period of small water resources in the drainage basin of Yenisei - Angara, which forced specific conditions of reservoir functioning. At rocky shores the predominating process is abrasion of their waterside parts, whereas at shores built of loose deposits the complete transformation of underwater parts of shallows takes place.