The systematization of natural water quality on the basis of various criteria promoted the develop-ment of various classifications of water impurity. Nowadays there is no uniform standard method of complex assessment of the impurity of water ob-jects. Thus single, indirect and complex indicators of pollution on hydrochemical and sanitary and bio-logical and other indicators are developed. The concept "complex assessment of impurity of natural water" is not standardized nowadays; this concept is accepted due to the need of reliable information about the level of cumulative pollution and has to give adequate assessment to the degree of impuri-ty of water objects. At natural measurements of water quality of water object a number of character-istics; cumulative assessment of quality of water demands the accounting of the contribution of each characteristic are received. One of possible solu-tions of this task of "convolution" of information is the creation of global scalar criterion (integrated criterion of quality). Complex assessment of water pollution of the reservoir cooler of BGRES-1 during various temporary periods of its functioning gives the chance to define natural phases of develop-ment of the ecosystem of water object since the period of filling, other periods of formation of eco-logical status; to develop recommendations on the improvement of water quality. Integrated ecological characteristic based on hydrochemical and hydro-biological indicators of water quality reflects difficult nature of interactions in natural and technogenic system "thermal power plant - environment". It al-lows objective estimating of general direction of change of hydrochemical and hydrobiological pa-rameters of water quality of reservoir cooler at vari-ous periods of its existence.