The paper presents the summary and the analysis of the results from a long-term monitoring of 90Sr specific activity in water of the lentic lakes Uruskul, L. Ighish, Kuyanysh and Travyanoye situated in Chelyabinsk Oblast on the East-Urals Radioactive Trace which was formed due to the 1957 accident at the Mayak PA. The data of measurements conducted over a 50-year period were approximated based on a function representing a sum of two (or three) exponents. The results of the analysis allowed us to study the peculiarities of reduction in the specific activity of 90Sr in the lakes of interest, and to obtain prognostic assessment of the development of the radioecological situation in these water reservoirs for the time period up to 2017. Presented in the paper are also data on the current levels of 90Sr and 137Cs specific activity in water of the lakes Uruskul, L. and S. Ighish, Kuyanysh, Travyanoye and Shablish. The estimates of the current reserves of these radionuclides are provided. Was shown that these lakes can be returned to the production use.