Abstract: The density of deposit in rivers increases under the action of water pressure and soil pressure,which results in large changes on the physical and mechanical properties of the soil. A large difference occurs when the threshold velocity is calculated by the empirical formula So,in this paper,the experiments on the threshold velocity for muddy loam are studied in special rectangular pipes. From the experiments,the frictional velocity is obtained at the critical condition Changed into the velocity for the natural river by resistance relationship and compared with the flow dynamic conditions of the river,it can be proved that the experimental results are effective and reasonable,providing a new method to solve the critical condition for the threshold velocity of deposit in rivers.