The recently frequent low water level in Lake Poyang attracted wide public attention. As an open lake, Lake Poyang is naturally connected with the Yangtze River via the Hukou Waterway. The interactions between the Yangtze River and the Lake Poyang are directly affected by the flow characteristics of the Hukou Waterway. The discharge capacity is introduced to represent the ability to convey water away using the hydraulic approach. The discharge capacity and its change is investigated here. Results show that:The discharge capacity was stable before 2000s and thereafter it rapidly increased. Its remarkable increase altered the interaction between the Yangtze River and Lake Poyang, leading to the recent severe drought regime. Large-scale sand dredging in Lake Poyang is attributed to be the cause of this increase. The management for sand dredging should be strengthened for conserving the water resources in Lake Poyang.