This study presents physical habitat simulations to assess the impact of hydropeaking on the downstream fish habitat. The study area is a 2.5 km long reach of the Dal River in Korea, located downstream from the Goesan Dam. Zacco platypus is selected as the target fish for the physical habitat simulations. The Goesan Dam releases high water discharges frequently for hydropower generations, which changes the water temperature in the downstream reach. This study investigates the impact of the temperature change on the fish habitat due to hydropeaking. Two periods of 6 days in winter and summer are considered separately. Hydraulic simulation is carried out using CCHE2D model, and distribution of water temperature is computed using CE-QUAL-W2 model. The Gene Expression Programming (GEP) model is used for habitat simulation. Water temperature is also used as physical habitat variables as well as flow depth, velocity, and substrate. Simulation results indicate that the physical habitat suitability decreases due to hydropeaking in both winter and summer. However, in winter, warm water discharge from the dam slightly increases the physical habitat suitability in such particular reach as the inlet and around the bend.