The quantification of the volume of water passing through a channel cross-section per unit time. Accurate flow measurement is the foundation of water resource management, flood forecasting, and environmental monitoring.
Flow measurement (also called streamflow or discharge measurement) is the determination of the volume of water flowing through a channel cross-section per unit time, expressed in cubic meters per second (m³/s) or cubic feet per second (cfs). The most common field methods include the velocity-area method (using current meters or ADCPs to measure velocity and depth across a cross-section), volumetric measurement (collecting a known volume over time for small flows), dilution gauging (injecting a tracer and measuring downstream concentration), and structure-based measurement using calibrated weirs, flumes, and rated control sections. Continuous flow monitoring at gauging stations relies on a stage-discharge relationship (rating curve) that converts automatically recorded water level (stage) to discharge. The USGS operates the largest streamflow monitoring network in the world, with over 13,000 stations providing real-time data essential for flood warning, water supply management, water rights administration, and ecological assessments. Measurement uncertainty is a critical consideration; even routine USGS discharge measurements have uncertainties of 3-5%, with higher uncertainty during extreme events. Emerging technologies include non-contact methods such as radar-based surface velocity measurement, image velocimetry using video analysis, and satellite-based discharge estimation that may eventually extend monitoring to ungauged river reaches.