The volume of water flowing through a river cross-section per unit time. It is the fundamental measurement of streamflow, typically expressed in cubic meters per second.
Discharge, or volumetric flow rate, is the quantity of water passing through a given cross-section of a stream, river, or channel per unit of time. It is calculated as the product of the cross-sectional area of flow and the mean velocity of the water. Discharge measurements are fundamental to nearly all aspects of hydrology, including flood forecasting, water supply planning, hydropower generation, and environmental flow assessment. Field measurement techniques include velocity-area methods using current meters or acoustic Doppler instruments, dilution gauging, and weir or flume installations. Continuous discharge records are typically derived from stage-discharge relationships called rating curves, where water level is continuously monitored and converted to discharge. The USGS maintains a network of over 10,000 stream gauges across the United States that provide real-time discharge data. Discharge data are used to construct flow duration curves, estimate flood frequencies, and assess the impacts of land use change and climate variability on water resources.