Dahua Reservoir is cated on the Hongshui River which forms the boundary between Dauan and Mashan Counties.When it has been completed,the reservoir will be the largest hydroelectric station in Guangxi. High temperature,abundant precipitation,massive and pure carbonate rocks, much folding and faulting are the major factors in the karst development of this area.Since the Quaternary Period,the land surface has been uplifted continuously and is now typical of the geomorphological landscape of karst“Fengcong”(peak cluster)and depression. Dolines are widely distributed within the karst depression.Underground water is usually below the level of the cave mouths,although it often emerges from the Caves during the flood season.This phenomenon suggests the possibility that there is an underground river.On the basis of the evidence provided by the distribution of water sinks and the dynamic behaviour of the water level,the water sinks were used to inject fluorescent dyes and coloured lycopodium spores at different times and at possible leakage points.Tracing experiments were conducted six times basically to establish the distribution of flow and the character of the underground river in this area.The results of the observations could be related to the dynamic state of the water level. We concluded that there is no leakage from the reservoir along underground passages as a result of the series of surveys and observations concerning the geological and geomorphological conditions,cave prospecting,long-term observations of underground water slnks,pumping tests,drilling exploration,as well as tracing experiments.This research provided a scientific basis for the construction of the reservoir.