The height of the water surface in a stream or river above a fixed reference datum. It is the most commonly measured hydrological variable and is used to estimate discharge.
Stage, also called gauge height or water level, is the elevation of the water surface in a stream, river, lake, or reservoir measured relative to a fixed datum point, typically an arbitrary local benchmark or a geodetic datum. Stage is the most fundamental and continuously measured hydrological variable because it can be recorded automatically using relatively simple and reliable instruments, including float-operated recorders, pressure transducers, and radar or ultrasonic sensors. Stage data are converted to discharge through a stage-discharge relationship (rating curve) that is established and maintained through periodic field measurements of both stage and discharge. The USGS and other agencies worldwide operate extensive networks of stream gauging stations that record stage at intervals of 15 minutes or less, providing real-time data for flood warning, water supply management, and environmental monitoring. Flood stage refers to the water level at which a stream overflows its natural banks and begins to cause damage. Stage records are among the longest continuous environmental datasets available, with some stations having over 100 years of observations.