The maximum instantaneous flow rate observed during a flood event at a specific point in a stream. It is the critical parameter for flood design and risk assessment.
Peak discharge is the maximum instantaneous rate of flow recorded at a stream gauging station during a flood event, representing the highest point on the storm hydrograph. It is the single most important parameter in flood hydrology because it determines the maximum water level, the extent of inundation, and the forces exerted on hydraulic structures. Peak discharge values are used in flood frequency analysis to estimate the probability of future flooding and to design bridges, culverts, dams, levees, and stormwater systems with appropriate safety margins. Estimation methods for ungauged catchments include regional regression equations developed by the USGS, the rational method for small urban watersheds, and the SCS curve number method. Peak discharge is influenced by rainfall intensity and duration, catchment area and shape, land use, soil type, antecedent moisture conditions, and channel characteristics. Climate change and urbanization generally increase peak discharge magnitudes by intensifying rainfall and reducing infiltration. The USGS StreamStats tool provides estimated peak discharge statistics for locations throughout the United States based on regional regression equations.