Precipitation regime and anthropogenic activities are the main factors that influence the river flow along the time. To assure the flow regulation and the flood mitigation some structural measures must be implemented. Building dams represents one of these measures. Their impact on the stream flow and climate change must be studied for avoiding negative effects on the environment. Therefore, our study focuses on the variability of the Buzau River (in Romania) water flow and its seasonal variation. The analysis is done for the period before and after the moment when the Siriu Dam (Romania) was built. The stationarity and the trend existence are analysed and the results are compared by statistical methods.