Increasingly, we are witnessing extreme hydrological events caused by various random natural phenomena. We cannot influence the random nature of the mechanisms, which trigger such events, but we can be partially prepared on them by analyzing past events. Floods represent such an event. Knowing the phenomena of high water allows a more efficient flood protection, appropriate land-use planning and an overall better water management. Frequency analysis of flood waves provides information on the relation between the value of the discharge and its return period. Some flood events are the result of multiple concurrent random events. Flooding on a river tributary, for example, can be influenced by both – discharge on that tributary and backwater effects due to discharge on the main stream. Coincident frequency analysis is used to develop a probability distribution for flood phenomena having no observed record when that phenomena is influenced by other random phenomena with available data. In the thesis we implemented a coincidence frequency analysis using the HEC-SSP program. We made the coincident frequency analysis of flood waves at the confluence of the Soča and the Vipava river, whereby we assessed the probability distribution of water levels of the Vipava river in Sovodnje on the basis of discharge data of the Vipava river in Miren and discharge data of the Soča river at the confluence. The water level of the Vipava river in its lower reach near the confluence is highly influenced by increased discharge of the Soča river at the confluence, which causes the backwater of the Vipava river. In order to reduce the probability of flooding of the Vipava river in its lower reach near the confluence, the stream of the Soča river should be first regulated in such a way, that at high water it would not cause the backwater of the Vipava river in such an extent.