This paper deals with the mathematical modeling of a water power plant. For the modeling procedure the plant was divided into logical parts and for each of these parts an appropriate mathematical model was developed. The interconnection of the different models is done by their common boundary conditions. The model also has to deal with a special type of an intersection between an open channel and a pipe flow. With suitable assumptions it is possible to simplify the model for this special kind of flow significantly. Finally, measurement results show the feasibility of the proposed assumptions for this plant.