Floods represent a phenomenon of rapid and significant increase and decrease of the levels, respectively the flows of the watercourses; these are defined by hydrographs. For the dimensioning, the execution and the exploitation of the hydrotechnical constructions in safe conditions it is necessary to know the flood waves with different probabilities. The paper presents the way of determining the flood waves with different probabilities for the cases when there is and when there is not enough data from measurements, followed by calculation examples. Finally, some flood prevention measures are recommended.