The measurement of stage (height) and, usually, computation of water flow in a stream or river. A streamgage is a hydrological monitoring installation at which a record of stage and, usually, flow (also called discharge) of a stream or river is obtained. Over 90% of the streamgages in the United States are operated by the Department of the Interior's U.S. Geological Survey (USGS). Currently, about 7800 streamgages are operated by the USGS, and over 90% provide real-time streamflow information that is published on the World Wide Web.
The streamflow information is used for many purposes, including flood planning and warning; streamflow forecasting; evaluation of the impact on streamflow from land use, water use, and climate; design of bridges, roads, culverts, water treatment plants, and navigation routes; water resource appraisal and allocations for water supply plans and interstate agreements; operation of locks and dams; power production; water-quality evaluations; habitat assessments; and recreation safety and enjoyment.
Streamgaging generally involves (1) obtaining a continuous record of stage (the elevation of the water surface at a location along a stream); (2) defining the current natural relation between stage and streamflow; and (3) using the stage–streamflow relation developed in step (2) to convert the continuously measured stage data into estimates of streamflow. Each of these three steps is explained in greater detail below.
A streamgage measures stage and consists of a structure that contains the instruments used to measure, record, and transmit stream-stage information (Figs. 1 and 2). A common approach to measuring stage (also called gage height) is with a “stilling well” in the river bank (Fig. 2) or attached to a bridge pier. Water from the river enters and leaves the stilling well though underwater pipes, allowing the water surface in the stilling well to be at the same elevation as the water surface in the river.
The stage is measured inside the stilling well using a float or an acoustic, optic, or pressure sensor. The measured stage value is stored in an electronic data recorder on a regular interval, usually every 15 min.
At other streamgages, stage is determined by measuring the pressure or weight of the water over an opening in a small tube at a fixed location in the stream. The measured pressure is related directly to the height of water over the fixed location in the stream.
Most users of streamflow information are interested in the volume and rate of water flowing in the stream or river, not just the water elevation. Continuously measuring streamflow is impractical. It can be estimated from relatively easy-to-measure stage, using the relation between stage and streamflow. The stage–streamflow relation depends on the shape, size, slope, and roughness of the channel and is different for every streamgage.
Stage–streamflow relations, often referred to as ratings, are developed for streamgages by measuring the flow of the river at different stages. An example stage–streamflow (discharge) relation is shown in Fig. 3. The development of a stage–streamflow relation requires numerous streamflow measurements at all ranges of streamflow.
Streamflow is the volume of water moving in a stream or river per unit of time, commonly expressed as cubic feet per second (ft3/s or cfs). In general, streamflow is computed by multiplying the area of water in a channel cross section by the average velocity of the water in that cross section; that is, streamflow = area × velocity.
The two most common instruments used for measuring velocity are the current meter and the acoustic Doppler current profiler.
Until recently, the most common device used for measuring stream velocity was the current meter in combination with a depth and stream-width measuring device. The stream channel cross section is divided into numerous vertical subsections (Fig. 4). In each subsection, the depth and width are obtained and the water velocity is determined by using a current meter (Fig. 5). The streamflow is computed by multiplying the subsection area (depth × width) and average velocity. The total streamflow is computed by summing the streamflow of each subsection.
The most commonly used current meter is the Price AA current meter (Fig. 5). The Price AA current meter has metal cups that rotate about a vertical axis. How fast the cups rotate is related directly to the velocity of the water. The Price AA meter can be attached to a wading rod or suspended just above a sounding weight in fast or deep water. In shallow water, the Pygmy Price current meter is used. A variation of the Price AA current meter is useful for measuring velocity under ice because its dimensions allow it to fit easily through a small hole. It has a polymer rotor to which ice and slush do not easily adhere (Fig. 6).
Acoustic Doppler current profiler
Advances in technology have allowed streamflow measurements by means of an acoustic Doppler current profiler (ADCP). An ADCP uses the principles of the Doppler effect to measure the velocity of water. The Doppler effect is the phenomenon we experience when passed by a car or train that is sounding its horn. As the car or train passes, the sound of the horn seems to drop in frequency. See also: Doppler effect
The ADCP is guided across the surface of a stream or river to obtain measurements of velocity and depth across the channel (Fig. 7). ADCPs use the Doppler effect to determine the water velocity by sending sound into the water and measuring the change in frequency of that sound reflected by sediment or other particulates in the water. The change in frequency that is measured by the ADCP can be translated into the velocity of the particle from which it reflected and the water velocity. ADCPs also use sound to measure water depth. Most commonly, GPS (Global Positioning System) is used to track the ADCP across the channel, providing channel width measurements. With the depth and width determined for calculating area and the velocity known across an entire channel cross section, the streamflow can be computed similar to the conventional current-meter method.
Measuring streamflow using acoustic instruments has become the most common method of measuring streamflow in the USGS. The USGS makes streamflow measurements at most streamgages every 6 to 8 weeks, making efforts to be sure that the full range of flow at the streamgage is measured regularly.
Converting stage information to streamflow information
At most streamgages, stage data are transmitted via satellite to USGS computers. Stage data are used to estimate streamflow using the developed stage–streamflow relation (rating) [Fig. 4]. The USGS has quality-control processes in place to help ensure that the streamflow information being reported across the country have comparable quality and are obtained and analyzed using consistent methods and techniques.
Most of the stage and streamflow information produced by the USGS are available in near real time on the World Wide Web (http://waterdata.usgs.gov/nwis). In addition to real-time streamgage data, NWISWeb (National Water Information System) provides access to daily and annual maximum streamflows for the period of record at all active and discontinued streamgages operated by the USGS.
Streamgaging involves obtaining a continuous record of stage, establishing and maintaining a relationship between stage and streamflow, and applying the stage–streamflow (discharge) relationship to the stage record to obtain a continuous record of streamflow. USGS streamflow information is critical for supporting water management, hazard management, environmental research, and infrastructure design. See also: Hydrology; River