Studies related to the prediction of floods have mainly focused on forecasting and protecting the flood time and area of the large river due to the increase of the slow flood level. The rise of temperature causes the El Niño and La Niña phenomena that abnormally change the temperature of seawater. Among natural disasters that lead to devastating damage, floods from torrential rains have been causing hundreds of victims and great loss of properties every year. This study is about calculation of peak velocity during a rainfall-runoff according at limitation rainfall runoff in the watershed forest by using GIS and Geomorphological Instantaneous Unit Hydrograph (GCIUH). In this paper, hydro-topographical property factors was acquired from GIS, limits depth of water was calculated by geomorphoclimatic instantaneous unit hydrograph and flood discharge during a rainfall-runoff was suggested by limits depth of water. When the depth of water is 50cm, the marginal flood is 96.5m / sec, it is 109.67m / sec at 70cm and 125.58m / sec at 100cm.