The Importance of Stream Gages
  1. Overview
  2. A Brief History of Stream Gages
  3. How Does Everyone Benefit From Stream Gages?
  4. How Are Data Collected and Transmitted?
  5. How Are Data Stored and Made Available to the Public?
  6. USGS Stream Gaging Stations in the Delaware River Basin
  7. Who Pays for the Maintenance of Gaging Stations?
  8. Threatened Stream Gages
  9. References

Water resources management has grown out of competition for the use of water and the fact that mother nature does not supply water at a constant or predictable rate. Whether for human supply, recreation, or habitat, there are many demands placed on this limited resource. In order to properly manage our water resources, we must have the means to measure them. Stream gages measure the quantity and variability of our surface water resources. In addition, the analysis of streamflow data in combination with groundwater data helps us understand the relationships of the hydrologic cycle. When flooding occurs, stream gages are indispensable as tools for flood forecasting and warning along rivers and streams.

The Delaware River Basin Commission (DRBC) believes that there is a need to increase awareness of the importance of stream gages. The purpose of these web pages is to provide an understanding of what stream gages do, how they work, and who is responsible for maintaining and funding them. Stream gages are a vital tool in our management of water resources. Operation of these gages under the exacting standards of the United States Geological Survey (USGS) provides a consistent set of streamflow data throughout the United States. Reliable funding of the stream gage network is necessary in order to obtain the information needed to make good decisions about the managing of our shared water resources.

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A Brief History of Stream Gages

The development of civilizations has always relied on the availability of fresh water. The United States is no exception. We have depended on our lakes, streams, and rivers to supply water and serve as lifelines allowing for growth, development, and expansion into new territory.

As a result of rapid growth in the 1880's, the U.S. population began to branch westward into the drier regions of the country, leaving the usually dependable waterways of the East far behind. Around the same time, John Wesley Powell, the second director of the USGS, requested that streamflow be monitored in eight river basins in the West. His goal was to measure the flow of streams and determine the potential for the irrigation systems that would be so vital to the economic development of this parched region. In 1889, the first U.S. stream-gaging station was established on the Rio Grande near Embudo, New Mexico. At this station, standard streamflow measurement procedures were devised.

Upon establishment of streamflow measurement methods at Embudo, personnel were dispersed to collect streamflow data at other western locations. Within two years the first streamflow measurements in the East were made on the Potomac River at Chain Bridge, near Washington, D.C., and a gaging station was established there on May 1, 1891. By 1895, discharge measurements were being made by the USGS in at least 27 states throughout the country.

Today, the USGS operates and maintains about 8,000 continuous-record stream-gaging stations in its network (the peak number in operation occurred in 1968 when there were 8,320 active stream gages).

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How Does Everyone Benefit From Stream Gages?

In the United States, there is an important resource which aids in preventing loss of life, saves one billion dollars a year in property losses, and allows bridges to be properly designed and drinking water allocated. Yet, it is a resource that the general public knows very little about. This resource is the USGS stream-gaging network.

This network provides the hydrologic information needed to aid in defining, using, and managing our country's invaluable surface water resources. The stream-gaging network provides a continuous source of well-archived, well-documented, and unbiased water data that are used in countless ways by governmental and private industries alike. The water data are put to work for everyone, every single day. Below are just some of the ways water data from the USGS stream-gaging network benefits all of us...

Flood Forecasting--This type of forecasting is a joint effort of several federal, state, and local agencies. Flood forecasting allows for timely warnings that save lives and decrease property damage. The National Weather Service (NWS), which is part of the National Oceanic and Atmospheric Administration, is the federal agency charged by law with the responsibility for issuing river forecasts and flood warnings. A reliable flood forecast, and its subsequent warning, depend on having a reliable and current source of depth and flow data, as well as precipitation data. Flood forecasts are based upon river models that provide estimates of how a river will respond to rainfall. River discharge and stage data provided by gaging stations are essential components of river models and flood forecasts. Although the NWS uses many sources of data to develop flood forecasts, the USGS stream-gaging network is the principal source of data on river depth and flow. Flood forecasts are prepared by 13 NWS river-forecast centers around the country.

Water Supply Allocation--Streamflow data assist water managers in making daily operational decisions for dealing with water requirements for municipal, industrial, and agricultural purposes, as well as demands for hydroelectric power generation and space for flood control in reservoirs. As of 1994, data from about 2,900 stations operated by the USGS were used by agencies, such as the Army Corps of Engineers, to operate more than 2,000 flood control, navigation, and water-supply reservoirs.

Highway Bridges--Many highway bridges are designed to withstand a 50- or 100-year flood (a flood event that has a 2% or 1% probability of occurring in any given year). Stream data from the USGS stream-gaging network assist in design decisions by providing the data needed to develop a design flood. A design flood that is too small could lead to a bridge design that causes water to back-up and inundate the road. Too large a design flood can lead to a design that is wasteful and requires a higher roadway than needed.

Wastewater Treatment Plants--Availability of long-term streamflows is important for wastewater treatment facilities. These facilities need to have estimates of streamflows available to allow for the proper dilution of treated wastewater releases. For example, if estimated low flows are understated, then there wouldn't be enough water available to properly dilute the wastewater. As a result, there would be a need for additional costly wastewater treatment to meet water-quality standards. USGS has archived its streamflow data since the gaging program began and it is a reliable source for such information.

Recreation--Stream gages don't just provide information useful to technical personnel and projects. Information about how high or low the water in a stream is or how fast the water is flowing is an important factor in recreation activities. Such information benefits countless outdoor enthusiasts, from canoeists and whitewater rafters, to fisherman and swimmers alike. Much of this information is now readily available on the World Wide Web for the general public to easily access.

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How Are Data Collected and Transmitted?

Stream Gage House.

A Gage House-the most common source of river stage data.

The data most often collected at a gaging station are stage. Stage is the height of the water surface above a reference elevation. The most common source of river stage measurement is a gage house (see diagram above). A gage house consists of a stilling well dug along the riverbank with a surrounding shelter that protects the equipment inside. Water enters the well through one or more inlet pipes. The water in the well rises to the same level as the river. Recording equipment in the gage house records the water level in the well (the stream stage). Recorded data can then either be accessed by telephone or, if the gaging station is equipped with an automated Data Collection Platform (DCP), it may be transmitted via satellite to agencies such as the National Weather Service and the Army Corps of Engineers. Data transmitted via satellite are a very important source of information for many agencies. Data are used to forecast river conditions, to issue flood warnings, and to plan reservoir releases or water withdrawals. The availability of this realtime data has become so important that the number of stations equipped with DCPs has been steadily increasing: there were 6,700 streamgages equipped with automated DCPs in 2007 compared to over 4,200 stations with DCPs in 1996.

Although stage information is useful for some purposes, most users of streamflow data find it necessary to have discharge information about a stream. Discharge is the volume of flow passing a specified point in a given interval of time and it is usually measured in cubic feet per second (cfs). Discharge includes not only the volume of water, but also any sediment or solids that may be mixed with or dissolved in the water. Unfortunately, providing discharge data is more difficult than providing stage data.

Discharge is usually estimated from stage/discharge relations known as rating curves. In order to develop a rating curve, USGS field personnel must visit a gaging station to measure river discharge and compare it to the stage. A Current Meter is the instrument that is frequently used to perform discharge measurements (see FIG. A & B). Year after year, discharge and stage data are gathered in the field, sometimes as often as once a month. Over time, a stage/discharge rating curve is gradually developed (see FIG. C). Unfortunately, rating curves are never fully complete due to the constant changes occurring in rivers and streams. Sedimentation, scour, changes in riverbed roughness, ice, debris or aquatic vegetation may significantly alter stage/discharge curves and must be adjusted for. In order to keep discharge estimates accurate, it is necessary for personnel to continue taking discharge and stage measurements in the field to keep the curves updated.

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How Are Data Stored and Made Available to the Public?

Before 1995, near realtime discharge data were stored in USGS databases and provided to major users via modem (telephone line) communications with USGS computers. Although data are still stored and available in USGS databases, advances in technology have made streamflow information more accessible to the general public.

In 1995, the USGS began to provide real-time discharge data to governmental agencies and the general public through the Internet. By 1996, hourly streamflow data were available for more than 2,600 sites. USGS reports that it currently serves real-time data for more than 9,900 sites and services up to 19 million requests per month. In addition to stream discharge data, hydrographs (graphs representing change in stage or discharge over a period of time) are also available for many stream-gaging stations. To view this information for yourself, click-on any one of the informative pages below:

Since its inception, the USGS has prepared annual reports listing all daily streamflow values at each gage. These reports are prepared for each state. Paper reports from years 1962 through 2005 are available in many libraries or can be purchased from the U.S. Government Printing Office. Some may also be available on-line at the USGS Publications Warehouse.

As of Water Year 2006 (data collected from October 1, 2005 onward), annual water data reports are available on the web as individual electronic Site Data Sheets. These sheets are available for stream and groundwater sites throughout the United States and can be viewed, retrieved, or printed from the web.

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USGS Stream Gaging Stations in the Delaware River Basin
  • As of April 2009, there were nearly 200 real-time stream gages in the Delaware River Basin (DRB). These gages are funded through federal, state, local, and private parties and are critical to such important functions as:
  • flood forecasting and warning;
  • water quality monitoring and management;
  • infrastructure construction (bridges, roads, culverts);
  • water system planning and management;
  • floodplain mapping; and
  • recreational activity management

While new gages have been added at different locations within the basin, many gages with long periods of record have been discontinued due to lack of funding. When a gage that has collected data for many years is permanently or even temporarily discontinued, a history of streamflow is fragmented and a valuable hydrological tool lost. This is a troubling circumstance as gages with long periods of streamflow information are vital to the effectiveness of the USGS stream-gaging program.

Long-record stream gages, defined by the United States Geological Survey (USGS) as stream gages with 30 or more consecutive years of record, are particularly valued for their long histories. Currently, the DRB has 108 long-record stream gages. This number represents a little more than half of the real-time gages currently operating in the basin.

The DRB is fortunate to have a number of stream gages that have recorded data since the early part of the last century. Approximately a dozen stream gages have 90 or more years of record, including the following:

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Who Pays for the Maintenance of Gaging Stations?

The USGS stream gage network is supported by four funding sources:

  • USGS Cooperative Water Program (CWP) - Funds made available by Congress to the USGS which require at least a 50% match from a partner in order for the USGS to invest the funds for stream gaging.
  • State and Local Funds - Partners in the USGS CWP.
  • USGS National Streamflow Information Program (NSIP)
  • Other Federal Agencies (OFA) - Primarily provided by the U.S. Army Corps of Engineers and Bureau of Reclamation to meet their water resources data needs.

State/local funds provided through the CWP and support from other federal agencies now account for nearly 70% of the USGS stream gage network funding (see chart below).

Funding for USGS Stream Gage Network
FY 2009 Total $146.3 Million

Funding for USGS Stream Gage Network, FY 2009 Total $146.3M

State/Local Funds = Cooperative Water Program-State and Local Contributions;
USGS-CWP = Cooperative Water Program-USGS Contributions;
USGS-NSIP = USGS National Streamflow Information Program;
OFA = Other Federal Agencies
Source: USGS


Currently, the USGS partners with over 850 state, and local partners through the Cooperative Water Program. Partners in this program usually fund individual stations in order to obtain data that meets an agency-specific project or regulatory need. Funding will be an extremely crucial issue for the future of the stream-gaging network. Over time, the cost-share for the USGS Cooperative Water Program has shifted from 50:50 to more like 70:30 (local share to federal share). In hard budget times, this level of cost share is not sustainable, which takes the form of local entities (states, local governments and others) decommissioning gages that have long continuous records. Stream gages are costly to maintain, and they need to be inspected/repaired as necessary. Rating curves must also be kept up-to-date with flow measurements. As new technology related to water-level sensors and data recorders emerges, funding will also be needed to replace outdated equipment.

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Threatened Stream Gages

Unfortunately, many gages with valuable long periods of record have been lost in recent years due to discontinued funding. Loss of long-record stream gages, defined by USGS as gages with 30 or more years of record, is on the rise. From 1995 to 2008, 948 critical stream gages with 30 or more years of record were discontinued across the country. In Water Year (WY) 2007 alone, more than 100 long-record gages were lost nationally. This represents about 2.5% of the total number of long-record gages operating in WY 2007. In the Delaware River Basin, a total of 16 stream gages were discontinued during the five year period 2002-2007. Five of these gages were long-record gages, two of which had more than 90 years of records.

USGS announced in March 2013 that it will discontinue operation of up to 375 stream gages nationwide due to budget cuts as a result of sequestration. Additional stream gages may be affected if partners reduce their funding to support USGS gages.

UPDATE (4/11/2013) Three continuous stream flow gages that were in danger of being discontinued in April 2013 will be funded by partner agencies and continue to operate:

UPDATE (4/11/2013) Two continuous stream flow gages were discontinued in April 2013 due to funding reductions from partner agencies (please note: all are located above New York City-Delaware Basin reservoirs and none are flood forecast locations):

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A New Evaluation of the USGS Streamgaging Network--A Report to Congress, U.S. Geological Survey, November 30, 1998.

Mason, Jr., Robert R., and T.H. Yorke, U.S. Geological Survey, Streamflow Information for the Nation, 1997.

Mason, Jr., Robert R., U.S. Geological Survey and Benjamin A. Weiger, National Weather Service, Stream Gaging and Flood Forecasting: A Partnership of the U.S. Geological Survey and the National Weather Service, 1995.

Middle Atlantic River Forecast Center web page, The Science of Hydrology, Everything You Ever Wanted To Know About River Forecasting (but were afraid to ask.), 1999.

Office of the Delaware River Master, for the photo of the stream-gaging station on the Delaware River at Montague, New Jersey.

Wahl, Kenneth L., Wilbert O. Thomas, Jr., and Robert M. Hirsch, The Stream-Gaging Program of the U.S. Geological Survey, U.S. Geological Survey Circular 1123, Reston, Virginia, 1995.

Water Resources of New Hampshire and Vermont Contoocook River "Visible" Gage Educational Site, New Hampshire/Vermont District USGS, Pembroke, NH, for photos of current meter and stage/discharge rating curve.

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