The level control in hydraulic power plants on rivers is of great importance for their automation. The level is controlled in several aims. The most important is to maintain the fall height at his maximum in order to maximise the produced energy. In this paper, a new modelling method of the flow dynamics is presented. The obtained model is especially suitable for small friction losses situations where the level waves phenomena can not be neglected. Then, this model is used to design the state-observer control. Finally, a practical case is presented.