In this paper, a hydraulic flood routing method for multibranch rivers is developed by means of the imaginary channel length using the double-sweeping method. This method is based on the St. Venant equations describing the channel flow movement and can solve some flood routing problems for which solutions cannot be obtained by the hydrological methods. The model presented in this paper provides an effective tool to handle the problem of flood routing for multibranch rivers and to treat the problem of weir control on flood movement.