USGS National Streamflow Information Program
People have used many ways to obtain estimates of the amount of water flowing in streams and rivers. Each of these ways can be considered a 'streamgage'. For example, some use a method to record the height of the water in the river and record that height periodically. An example of this method would be a 'staff gage' (see below). Another type of streamgage is one that only provides information on the highest flow since the gage was last visited. This type of streamgage is called a 'crest stage gage' (see below).
Another type of streamgage is one that is designed for warning of floods only. This type of streamgage is typically referred to as an 'ALERT' (Automated Local Evaluation in Real-Time) streamgage and it is designed to send a warning when the water level reaches a predetermined level or changes rapidly.
'Partial record streamgages' are those operated and quality assured only for given flow regimes such as high flow, peak flow, low flow, etc. The period (or date) of the quality assured flow value(s) may be variable from one year to another. These streamgages could include crest stage gages and low flow correlation sites. Seasonal streamgages are those that are operated and the data is quality assured for a defined period of the water year (i.e. a streamgage which is only operated during irrigation allotment season).
Because there are many different streamgages, the USGS has developed a very specific definition of what is meant by 'streamgage' for the purposes of the National Streamflow Information Program and these web pages - unless otherwise stated, "streamgage" is an active, continuously functioning measuring device in the field for which a mean daily streamflow is computed or estimated and quality assured for at least 355 days of a water year or a complete set of unit values are computed or estimated and quality assured for at least 355 days of a water year.
Most USGS streamgages operate by measuring the elevation of the water in the river or stream and then converting the water elevation (called 'stage') to a streamflow ('discharge') by using a curve that relates the elevation to a set of actual discharge measurements. This is done because currently the technology is not available to measure the flow of the water accurately enough directly.
The USGS standard is to measure river stage to 0.01 inches. This is accomplished by the use of floats inside a stilling well, by the use of pressure transducers that measure how much pressure is required to a push a gas bubble through a tube (related to the depth of water), or with radar.
At most USGS streamgages, the stage is measured every 15 minutes and the data is stored in an electronic data recorder, most often powered by solar energy. At set intervals, usually between every 1 to 4 hours, the data is transmitted to the USGS using satellite, phone, or radio. At the USGS offices, the curves relating stage to streamflow are applied to determine estimates of the streamflow and both the stage and streamflow data are then displayed on the USGS web pages.
For more information on how streamgages work, see: http://pubs.usgs.gov/fs/2005/3131/.
For more information on real-time streamflow data, see: http://pubs.usgs.gov/fs/2007/3043/.