The article presents the development of a mathematical model of the system dynamics. Mathematical model allows forecasting the characteristics of flood effects. Model is based on a causal diagram and is presented by a system of nonlinear differential equations. Simulated characteristics are the nodes of the diagram, and edges define the functional relationships between them. The numerical solution of the system of equations using the Runge-Kutta method was obtained. Сomputer experiments to determine the characteristics on different time interval have been made and results of experiments have been compared with real data of real flood. The obtained results make it possible to assert that the developed model is valid. The results of study are useful in development of an information system for the operating and dispatching staff of the Ministry of the Russian Federation for Civil Defence, Emergencies and Elimination of Consequences of Natural Disasters (EMERCOM).