In mountainous rivers,dredging is frequently employed to increase channel depth.However,long-distance channel excavation often induces water level degradation,thereby altering originally designed navigable water levels.Research on variations in navigable water levels is carried out based on analysis of measured hydrological data and numerical simulation.The results indicate that,for the Qujiang and Lanjiang waterways,the designed minimum navigable water level adopts a 98% guarantee rate.After 2010,the water level with a 98% guarantee rate decreased slightly by 0.14 m,still maintaining a 0.34 m margin compared with the current designed minimum navigable water level in this section.The designed maximum navigable water level is determined as the water level corresponding to the flow rate that ensures the safe navigation of ships.The maximum navigable flow rate of the Lanjiang River is 5,340 m3/s,with a corresponding water level of 27.6 m.During the same period,the designed maximum navigable water level at the Lanxi hydrological station dropped by 1.0 m.Under the condition of the same flow rate,the water level decrease has caused an increase in longitudinal flow velocity in the channel section of bridge zones,which is unfavorable to the navigation safety of ships.The class III channel improvement project involves a dredging length of 40 km.After the project,the maximum water level drop at the downstream approach channel of Yaojia lock is about 0.1 m,while the water level at Lanxi hydrological station (the terminal of the dredging section) remains basically unchanged.