When a flood occurs it is essential in order to estimate the damage caused by it the depth of water is calculated. Use hydraulic methods for flood routing and calculation of water depth due to the need for river sections at appropriate distances it is time-consuming, difficult and costly. The use of hydrological methods such as the linear Muskingum method for flood routing, in addition to its simplicity and low cost, is also good accuracy. So far, the linear Muskingum method has been used to calculate the outflow hydrograph while in economic analysis and flood damage estimation and flood management and engineering, calculation of depth changes compared to time is of great importance. Therefore, in the present study, using the linear Muskingum method and the PSO algorithm instead of calculating the variation of the outflow discharge compared to time, the variation in the depth of water output compared to the time interval between the hydrometric stations of Mollasani and Ahwaz of the Karun River have been calculated. The new method presented in this study, in addition to being of good accuracy, is simpler and less costly than the hydraulic methods.