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Article highlights This work improves the Preissmann implicit difference method to effectively simulate the propagation characteristics of unsteady flow in long reaches. The daily adjustment of unsteady flow in the upper reaches increases the river flow during the low-water period so that the designed water level under the action of this daily unsteady flow adjustment increases by approximately 0.19 m compared to that under normal conditions. A large number of point bars have been damaged in the wake of sand excavation in long reaches,resulting in a water level decline of approximately 0.5–2.0 m.