Abstract: In the tidal rivers, determination of salinity concentration is a complex problem. Tidal surges and fresh water influence the tidal river. These factors are two independent phenomena. These phenomena are combined by joint probability method. Salinity concentration depends on tidal height in the mouth of the tidal rivers and discharge of fresh water in the upstream of the tidal rivers. Distribution coefficient of no tidal rivers depends on velocity of current, radius of hydraulic and Manning`s coefficient. In addition to above factors, distribution coefficient of tidal rivers depends on domain of tidal surge, period tidal surge and the discharge of fresh water. The Karun River in Iran was selected for case study. This river is the most important tidal river in Iran. By joint probability method, the combination of tidal surge and discharge of fresh water that produces maximum salinity concentration is selected for each section of the tidal rivers.