The article considers the development of algorithms and models for determining the probability of an adverse combination of events affecting the operational safety of a hydroelectric power plant. While developing the mathematical support, the formal apparatus of the Markov processes theory was used, thus making it possible to determine the occurrence probability of unfavorable combinations of events, each of which separately does not have a significant effect on the safety of the operation of the HPP from the solution of the Kolmogorov-Chapman system of differential equations. The results obtained can be used in the development of DSS.