A stream gage refers to a site along a stream where measurements of volumetric discharge (flow) are made. In the United States, the U.S. Geological Survey (USGS) is the principal federal agency tasked with maintaining records of natural resources. Within the USGS, Water Resources Division carries the responsibility for monitoring water resources. The United States Geological Survey (USGS) is a scientific agency of the United States government. ...

To establish a stream gage, USGS personnel first choose a site on a stream where the geometry is relatively stable. Many times this will be at a bridge or other stream crossing. They then install equipment that measures the stage, or elevation of the water surface or, more rarely, the velocity of the flow. Additional equipment is installed to record these values and telemeter the readings to the district office where the records are kept.

In the case of stage measurements, a rating curve must be constructed. A rating curve is the functional relation between stage and discharge. It is determined by making repeated measurements of flow using a current meter or other velocity measuring device. The technicians and hydrologists responsible for determining the rating curve visit the site during flood events (in particular) and make a discharge determination by following an explicit set of instructions, called a discharge measurement.

Once the rating curve is established, it can be used in conjunction with stage measurements to determine the volumetric discharge of the stream. This record, then, serves as an assessment of the volume of water that passes by the stream gage and is useful for many tasks associated with hydrology.

The other approach, called an index velocity determination uses a velocity meter, often either magnetic or acoustic, to measure the velocity of the flow. A rating curve, similar to that used for stage-discharge determinations, is constructed using discharge determinations to relate the indicated flow velocity with a stream discharge. Then these data can be used analogously to those from the traditional stream gaging method.