Due to geographical features of being close to DPRK (Democratic People’s Republic of Korea), the Imjin River basin has difficulties in hydrological observation, and is vulnerable to unexpected flood occurrence. As a countermeasure, Gunnam Flood Control Reservoir construction was planned in 2005. Despite such a structural measure, damages by DPRK’s illegal release continues to occur. Futhermore the Imjin River’s flow has been decreased due to the effect of continuous drought in the Korean Peninsula since 2012 and DPRK’s unilateral storage of water. A new operation method is derived for the Gunnam Flood Control Reservoir in order to cope with drought damages on the Imjin River basin and to ensure efficient response time upon flooding. The operation method maintaining Gunnam Flood Control Reservoir’s water level by raising from EL.23.0 m to EL.31.0 m during the flood season for securing reservoir capacity enables to secure additional 14,000,000 m3 water compared to the existing operation methods. The operation method to store inflow by controlling release to 250 m3/s in the early stage of flood has increased 2.66% on average in terms of detention effect of reservoir compared to the existing operation methods. The method enables to secure 19 hours to prepare flood compared to the existing methods.