Stream-Gaging Program of the U.S. Geological Survey
Reston, Virginia, 1995
By Kenneth L. Wahl, Wilbert O. Thomas, Jr., and Robert M. Hirsch


The basic piece of data obtained at a station is the stage, which is the height of the water surface above a reference elevation. If the stage of the streambed is known and is subtracted from the water-surface stage, then the result is the depth of water in the stream. Although stage of a stream is useful in itself in planning uses of flood plains, most users of streamflow data need to know the discharge of the stream. Discharge is defined as the volume of flow passing a specified point in a given interval of time and includes the volume of the water and any sediment or other solids that may be dissolved or mixed with the water. The units of discharge usually are measured in cubic feet per second (or cubic meters per second, if metric units are used). Discharge is derived from the stage data through the use of a relation between stage and discharge. The stage-discharge relation for a specific stream location is defined from periodic discharge measurements made at known stages.

Standard methods of data collection are used as described by Rantz and others ( 1982 ) and in the publication series Techniques of Water-Resources Investigations of the U.S. Geological Survey. Those methods are briefly described in the following sections.

Measuring Stage

Perhaps the most common method of measuring the stage of a river is through the use of a stilling well. Stilling wells are located on the bank of a stream or on a bridge pier and are topped by a shelter that holds recorders and other instruments associated with the station. The well is connected to the stream by several intakes such that when the water level changes in the stream, the level simultaneously changes in the well (fig. 10). Thus, the water surface in the well is maintained at the same level (stage) as the water surface in the stream. The well damps out the momentary fluctuations in the water surface in the stream due to waves and surging action that may be present in the river. An outside reference gage, typically a graduated staff gage, is read periodically to verify that the water level in the well is indeed the same as the water level in the stream and that the intakes are not plugged. As the water level in the well rises or falls, a float in the well also rises or falls. A graduated tape or beaded cable attached to the float and with a counterweight on the other end is hung over a pulley. This pulley drives a recording device. Historically, the recording device would have used a pen that recorded a graph of the river stage as it changed with time. Graphic recorders are still in use; today, however, the stage is more commonly recorded on a punched paper tape or an electronic recorder or is transmitted to the office by means of satellite.

Figure 10. Schematic of a stilling well and shelter.

Cover Photo 1 (70Kb GIF) Stilling well and shelter for measuring the stage of a river.

In many cases, stilling wells are impractical because of difficulties either in installation or operation. Stations that use a bubbler system are an alternative because the shelter and recorders can be located hundreds of feet from the stream. In a bubbler system, an orifice is attached securely below the water surface and connected to the instrumentation by a length of tubing. Pressurized gas (usually nitrogen or air) is forced through the tubing and out the orifice. Because the pressure in the tubing is a function of the depth of water over the orifice, a change in the stage of the river produces a corresponding change in pressure in the tubing. Changes in the pressure in the tubing are recorded and are converted to a record of the river stage.

Measuring Discharge

The most practical method of measuring the discharge of a stream is through the velocity-area method. This method requires the physical measurement of the cross-sectional area and the velocity of the flowing water. Discharge is determined as the product of the area times the velocity. Velocity is measured by using a current meter. The meter consists of a propeller that is rotated by the action of flowing water. The rotation depends on the velocity of the water passing by the propeller. With each complete rotation, an electrical circuit is completed and recorded in some fashion. Given the number of revolutions in a given time interval, velocity can be determined for the location of the current meter.

Photo 1 (92Kb). Current meter and weight suspended from a bridge crane.

Measuring the average velocity of an entire cross section is impractical, so the method uses an incremental method. The width of the stream is divided into a number of increments; the size of the increments depends on the depth and velocity of the stream. The purpose is to divide the section into about 25 increments with approximately equal discharges. For each incremental width, the stream depth and average velocity of flow are measured. For each incremental width, the meter is placed at a depth where average velocity is expected to occur. That depth has been determined to be about 0.6 of the distance from the water surface to the streambed when depths are shallow. When depths are large, the average velocity is best represented by averaging velocity readings at 0.2 and 0.8 of the distance from the water surface to the streambed. The product of the width, depth, and velocity of the section is the discharge through that increment of the cross section. The total of the incremental section discharges equals the discharge of the river.

Photo 2 (81Kb). Wading rod and current meter used for measuring the discharge of a river.

Photo 3 (40Kb). Crane, current meter, and weight used for measuring the discharge of a river from a bridge.

When the stage is low and the stream can be waded, the measurements are made by wading with the current meter mounted on a wading rod. The meter is positioned at the appropriate depth on the wading rod, which also is used to measure the water depth. If the water is too deep for wading, then the measurement is made either from a bridge or cableway across the stream. If the measurement is made from a bridge or cableway, then the meter is suspended on a thin cable wound on a reel. A torpedo-shaped weight is attached below the meter to permit it to be lowered into the water and to hold it in position once submerged. If measuring from a bridge, then the reel is mounted on a wheeled frame (or crane) that permits the lowering of the meter assembly over the bridge rail; from a cableway, the reel is mounted in a cable car suspended from the cableway that crosses the river.

Cover Photo 2 (50Kb GIF). Cable car, current meter, and weights used for measuring the discharge of a river from a cableway.

The basic procedure of measuring width and velocity is the same, however, whether the measurement is made by wading or from a cableway or bridge. The USGS makes more than 60,000 discharge measurements each year. The distribution of measurements made in 1993 is shown in figure 11.

Figure 11.Discharge measurements made in 1993, by State or possession.

Determining a Continuous Record of Discharge

Measurements made over the range in stage of the stream are plotted against the corresponding stages to define the stage-discharge relation that is used in conjunction with the recorded stage record to determine the discharges throughout the year. The procedure would be fairly straightforward were it not for all the natural processes that occur in streams. Flowing water moves sediment and other material that if eroded from or deposited on the streambed or banks, can alter the cross-sectional area of the stream at a given stage. Growth of vegetation along the banks and aquatic growth in the channel itself can impede the velocity, as can deposition of downed trees in the channel. Processes like these will alter the stage-discharge relations and are characteristic of most streams. In addition, ice and snow can produce large changes in stage-discharge realtions, and the degree of change can vary dramatically with time.

The stage-discharge relation will be stable if the hydraulic characteristics of the general reach of stream are unchanging and the bed material does not move appreciably. On a stable stream, periodic measurements are made every 6 to 8 weeks to verify that the relation has not undergone some unrecognized change. The stage-discharge relation will be unstable, changing with time and with the flow conditions, if the streambed or the hydraulic roughness is changing (as might occur with a sand-bed stream). In such cases, frequent measurements (about weekly) are needed to define how the rating is changing and to define its present condition (fig. 12).

Figure 12
. Sections of stage-discharge relations for the Colorado River at the Colorado--Utah State line.

Sometimes, current-meter measurements are not possible during large floods. However, the stage and discharge of those floods are essential in defining the rating for the range of flow. Therefore, the discharge is determined indirectly by surveying the high-water marks left by the flood and by using hydraulic formulas to calculate the discharge for the peak stage.

Because the relation between stage and discharge may vary with time, the discharge is known only with certainty at the time of discharge measurements. If the relation is changing, then judgement must be used to determine the most probable status of the stage-discharge relation for times between discharge measurements. In fact, changes in the stage-discharge relation may not be evident until a whole series of measurements are available for analysis. Therefore, the computational process usually goes through the following steps:

1. Following a measurement, a preliminary evaluation is made of the degree to which the stage-discharge relation has changed on the basis of measurements made up to that time. Provisional discharges are determined, assuming that the most recent measurements define the channel condition.
2. This process is repeated following each measurement. However, with each measurement, more measurements are available to evaluate the stage-discharge relation. This may lead to changes in the provisional discharges that had been computed for previous months.
3. At the end of the year, all measurements are available for review. The entire set of measurements are used to reevaluate the rating conditions for the year. Final decisions are made about the stage-discharge relation that were in effect during the year and the record is refined or recomputed as necessary. This record is then passed through a rigorous review process and, once approved, the data are considered final and are placed in the archives and published.

Data Collected by Other Agencies

Other agencies, most notably State agencies, collect some streamflow data, as do a number of cities, local governments, and other Federal agencies. The primary differences between the USGS networks and those of the other agencies are the purposes for which the data are collected. Other agencies, whether they are Federal, State, or local, often collect only those parts of the data needed for a specific mission or task. For example, data collected by other agencies to fulfill permitting requirements associated with wastewater or treated water commonly do not include the full range of flows. These data, while vital for that specific mission, generally have little transfer value and are, therefore, of limited value in addressing issues of national and regional scope ( Hren and others, 1987; Childress and others, 1989). Consequently, these data are not usually placed in accessible archives and made readily available to others.

Some data collected by other agencies, however, have value beyond the specific purpose for which the data were collected. Data from stations operated by other agencies are reviewed by the USGS, published in the annual State Water Data Reports series compiled by the USGS and entered in the USGS data base (see fig. 9). In 1990, data from about 400 stations were provided to the USGS by other agencies.

Dissemination of Data

Currently, daily-discharge data are published on a water-year basis for each State in the USGS report series Water Resources Data---[State Name]. A water year is the 12-month period from October 1 through September 30 and is designated by the calendar year in which it ends. Thus, the 1994 water year ends September 30, 1994. Because of the need for review of the completed computations, these reports generally are published from 6 months to 1 year after the end of the water year. The present report series, in which the data are released on a state-boundary basis, began with the 1971 water year.

Before the introduction of the present publication series, water-resources data were published in USGS Water-Supply Papers. Before September 1960, data for each major river basin were published in annual Water-Supply Papers. From 1961 through 1970, the data for the major river basins were published in 5-year summaries.

Many streamflow-data users must make operational decisions daily. For these users, streamflow records are computed and made available on a provisional basis. Today, more than one-half of the currently operating stations have equipment that permits immediate transmission of data by means of satellite from the data-collection site. By using the telemetry, data are transmitted around the clock by means of two geostationary operations environmental satellites (GOES) that are positioned above the Earth at an altitude 22,300 miles above the Equator over the eastern Pacific Ocean and Brazil. The satellites are operated by the National Oceanic and Atmospheric Administration. These data then are retransmitted by means of a domestic satellite, and the resulting signal is received by the USGS and other users (fig. 13). The transmission and receipt of the signals are automated, as are the provisional discharge computations that are available for meeting current data needs.

Automated telemetry provides the water-data users with provisional stage and discharge information in a timeframe that meets water-management needs. This technology permits the USGS field offices to monitor the operation of the hydrologic stations continuously, time visits to stations to coincide with times of maximum need for data (such as during floods), and to service equipment at the stations.

In addition to the published record, the data collected by the USGS are archived in the National Water Data Storage and Retrieval System, which is a computerized data base widely known by the acronym WATSTORE ( Hutchinson, 1975 ). The WATSTORE system contains the data and a number of programs that can be used to analyze and produce statistical summaries of the data contained therein.

Figure 13.
Figure 13.
Schematic of the data flow for realtime operations.

Beginning with the 1990 water year, Water Data Reports also are available on Compact Disk--Read Only Memory (CD--ROM). The Water-Supply Papers, the Water Data Reports, and the CD--ROM's are distributed to participating agencies and libraries; they also are available for sale by the USGS Earth Science Information Center, Denver, Colorado. The USGS currently is developing procedures to allow access to streamflow data by means of Internet. Historical mean daily-discharge data for about 18,500 stations will soon be available through this source. The USGS "Home Page" on the World Wide Web is

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Stream-Gaging Program of the U.S. Geological Survey
Reston, Virginia, 1995
By Kenneth L. Wahl, Wilbert O. Thomas, Jr., and Robert M. Hirsch

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