A streamgage is a structure installed beside a stream or river that contains equipment that measures and records the water level (called gage height or stage) of the stream. Streamflow (also called discharge) is computed from measured water levels using a site-specific relation (called a stage-discharge rating curve) developed from onsite water level and streamflow measurements made by USGS hydrographers. The water level and streamflow data are quality assured and made available online.
Most USGS streamgages are used to generate continuous streamflow information year-round. Together these gages constitute the National Streamflow Network. Some gages, however, only record the water level (gage height or stage) of a stream, lake or reservoir; no streamflow (discharge) is computed. These less costly gages are used to observe trends in water levels, when flow information is not needed or not applicable (lakes). Other streamgages, known as partial-record gages, are operated and quality assured only for given flow regimes such as high flow, peak flow, low flow, etc. These streamgages include crest-stage gages that are used to determine the elevation of a peak stage occurring at a specific location in a stream—used for flood-frequency analyses. Seasonal streamgages are those that are operated for a defined period of the water year (for example, a streamgage that is only operated during irrigation allotment season). Rapid-deployment gages (RDGs) are gages that are temporarily fixed on structures above streams and rivers (such as bridges) during emergencies to provide water level information when a streamgage does not exist or is damaged. Supergages are a small subset of streamgages that collect both streamflow and continuous water-quality data.
Streamgages do not measure streamflow directly. Rather, streamflow monitoring involves several steps, which generally include (1) continuously measuring and recording the stage of a stream, (2) periodically taking discharge measurements in the stream, (3) developing the relation between stage and discharge for the site and applying this relation to the continuous stage record to compute streamflow (discharge).
Stage Measurement
Stream stage can be measured using a variety of methods. These methods include the traditional float/stilling-well method and newer methods that utilize pressure transducers, gas-purge (bubbler) systems or radar sensors, etc. Stream stage is usually expressed in feet and hundredths of a foot, or in meters and hundredths or thousandths of a meter. The accuracy of the measurements generally is ±0.01 foot or 0.2 percent of the effective stage.
A streamgage usually measures stage every 15 minutes. When intense rainfall and runoff cause a stream or river to rise quickly, however, the time intervals can be as short as every 5 minutes. The data are typically transmitted to USGS computers on a preset schedule by way of satellite—usually every 1 to 4 hours. During high water or other emergency situations, however, data will be sent to the satellite every 15 minutes to provide more timely information.
Discharge Measurement
Streamflow, or discharge, is the volume of water moving down a stream or river per unit of time, commonly expressed in cubic feet per second or gallons per day. Because stream discharge cannot be measured directly, it must be computed from variables that can be measured directly, such as stream depth, stream width, and streamflow velocity. This is generally done by multiplying the cross-sectional area of water in the stream channel by the average velocity of the water in that cross section (see below). Although streamflow is computed from measurements of other variables, the term “streamflow measurement” or “discharge measurement” is generally applied to the final result of the calculations.
There are a variety of methods for measuring streamflow velocity. Two of the more common methods are the mechanical current-meter method and the Acoustic Doppler Current Profiler (ADCP) method. In the mechanical current-meter method, the stream channel cross section is divided into vertical subsections and a current meter is used to estimate the velocity in each subsection. As water flows past the current meter, a wheel of metal cups revolves around a vertical axis and transmits an electronic signal on each revolution, allowing the revolutions to be counted and timed. The rate at which the cups revolve is translated into water velocity. Subsection velocities are multiplied by corresponding subsection areas; these values are summed to compute total discharge for the cross section. In contrast, ADCPs are hydroacoustic instruments that use the principles of the Doppler Effect to measure water velocity by sending a sound pulse into the water and measuring the change in the frequency of the sound pulse reflected back to the ADCP by sediment or other particulates being transported in the water. The change in frequency, or Doppler Shift, that is measured by the ADCP is translated into water velocity. The ADCP provides a detailed profile of water velocity and direction for the majority of a cross section, rather than limiting measurements to point locations within subsections of the stream channel. This improves discharge measurement accuracy.
