Frequently Asked Questions - Real-Time Streamflow Data
Questions and answers are original compositions or are compiled from any available sources and credit is given where appropriate. New material will be added as needed. Contributions are welcome. Be sure to visit our Outreach pages for more information about who we are and what we do at the Maryland, Delaware, District of Columbia District of the USGS, Water Resources Division.
Real-time Streamflow Data
- General Information
- Why does the USGS collect streamflow data?
- How does the USGS collect streamflow data?
- River and Reservoir Forecasts
- How can I obtain river forecasts?
- How can I obtain reservoir release schedules?
- Data Availability
- How can I obtain real-time streamflow information?
- How can I obtain real-time reservoir information?
- How are stations selected for real-time data services?
- Why are updated data available only every 4 hours?
- Why are data sometimes not available for a particular stream-gaging station?
- Why do some real-time stations experience equipment problems for extended periods of time?
- Data Accuracy and Quality
- Why might USGS streamflow data reports not be accurate?
- Why are there sometimes differences between river stages as reported by the National Weather Service and by USGS?
- I frequently kayak the Potomac River and the real-time stage data reported by USGS seem to be too high (or too low). Are the USGS data inaccurate?
- Why is the discharge reported at a particular stage sometimes different in different years or at certain times in the same year?
General Information
Why does the USGS collect streamflow data?
The U.S. Geological Survey (USGS) stream-gaging program provides streamflow data for a variety of purposes that range from current needs, such as flood forecasting, to future or long-term needs, such as detection of changes in streamflow due to human activities or global warming. The development of data on the flow of the Nation's rivers mirrors the development of the country. From the establishment of the first stream-gaging station operated by the USGS in 1889, this program has grown to include 7,292 stations in operation as of 1994. Data from the active stations, as well as from discontinued stations, are stored in a computer data base that currently holds mean daily-discharge data for about 18,500 locations and more than 400,000 station-years of record. The stream-discharge data base is an ever-growing resource for water-resources planning and design, hydrologic research, and operation of water-resources projects.
The USGS stream-gaging program provides hydrologic information needed to help define, use, and manage the Nation's water resources. The program provides a continuous, well-documented, well-archived, unbiased, and broad-based source of reliable and consistent water data. Because of the nationally consistent, prescribed standards by which the data are collected and processed, the data from individual stations are commonly used for purposes beyond the original purpose for an individual station. Those possible uses include the following:
Enhancing the public safety by providing data for forecasting and managing floods
Characterizing current water-quality conditions
Determining input rates of various pollutants into lakes, reservoirs, or estuaries
Computing the loads of sediment and chemical constituents
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Understanding the biological effects of contamination
Delineating and managing flood plains
Setting permit requirements for discharge of treated wastewater
Designing highway bridges and culverts
Setting minimum flow requirements for meeting aquatic life goals
Monitoring compliance with minimum flow requirements
Developing or operating recreation facilities
Scheduling power production
Designing, operating, and maintaining navigation facilities
Allocating water for municipal, industrial, and irrigation uses
Administering compacts or resolving conflicts on interstate rivers
Defining and apportioning the water resources at our international borders
Evaluating surface- and ground-water interaction
Undertaking scientific studies of long-term changes in the hydrologic cycle
Data for one or more of these purposes are needed at some point in time on virtually every stream in the country, and a data-collection system must be in place to provide the required information. The general objective of the stream-gaging program is to provide information on or to develop estimates of flow characteristics at any point on any stream. Streamflow data are needed for immediate decision making and future planning and project design. Data, such as that needed to issue and update flood forecasts, are referred to as "data for current needs." Other data, such as that needed for the design of a future, but currently unplanned, bridge or reservoir or development of basinwide pollution control plans, are referred to as "data for future or long-term needs." Some data, of course, fit into both classifications; for example, a station that supplies data for flood forecasting and also provides data to define long-term trends.
Reference: Wahl, Thomas, and Hirsch, 1995
How does the USGS collect streamflow data?
USGS has used consistent procedures for over 100 years to collect streamflow data, and currently operates over 7000 stream-gaging stations across the United States. All regular stream-gaging stations record stage, or water depth, at a fixed time interval, usually every 15, 30, or 60 minutes. Most stations in Maryland, Delaware, and the District of Columbia record at a 15-minute interval. Stage is most often measured using either a float and pulley device or a pressure transducer.
A relationship is developed by USGS hydrographers between stage (usually expressed as feet) and discharge (usually expressed as cubic feet per second). This relationship is developed by making frequent direct discharge measurements at stream-gaging stations. Discharge is the primary streamflow data product of USGS, and operations have been specifically designed to provide accurate determinations of average daily streamflow, flood peaks, minimum flows, and flows associated with water-quality samples. In most cases, USGS operations have not been designed or funded to provide accurate instantaneous streamflow data, although customer demand for these data is increasing. In any case, the accuracy of instantaneous streamflow data reported by USGS is acceptable for developing the principal data products and for most other applications. The accuracy of data may not be sufficient for applications requiring high resolution.
All USGS streamflow data, including real-time data, are PROVISIONAL and subject to revision until published in the annual Water-Data Report.
River and Reservoir Forecasts
How can I obtain river forecasts?
River forecasts are made by the National Weather Service River Forecast Centers and released through local Weather Service Offices. The vast majority of current streamflow data used for these forecasts is obtained from U.S. Geological Survey stream-gaging stations, but USGS has no other responsibility or authority for making forecasts. More information for Maryland, Delaware, and D.C. can be obtained from the Mid-Atlantic River Forecast Center in State College, PA and the Washington-Baltimore Weather Service Office in Sterling, Virginia.
How can I obtain reservoir release schedules?
Reservoirs are managed by agencies other than USGS. USGS has no water resources management or regulatory responsibility or authority. Many major reservoirs in the mid-Atlantic area are managed by the U.S. Army Corps of Engineers. Release schedules can be obtained via their web pages for the Delaware, Potomac and Susquehanna, and the Youghiogheny river basins.
Data Availability
How can I obtain real-time streamflow information?
Real-time streamflow data are available from the U.S. Geological Survey for over 4200 stations throughout the United States. Data are available for about 35 stations in Maryland and Delaware. These data are available only through the World-Wide Web.
The National Weather Service provides stage data at selected stations at least twice daily via their web pages for the Potomac and Rappahannock River Basins and Monongahela, Susquehanna, and Delaware River Basins. There is also a telephone recording provided for the Potomac and Rappahannock River Basins at (703)260-0305.
The American Whitewater Affiliation provides a compilation of web pages and telephone numbers where real-time streamflow and reservoir information can be obtained across the United States.
Note that direct telephone access to U.S. Geological Survey stream-gaging stations is not authorized except for official use, including those stations where National Weather Service equipment is co-located.
Access to these stations must be restricted to official use so that data are available during emergencies.
How can I obtain real-time reservoir information?
Reservoirs are managed by agencies other than USGS. USGS has no water resources management or regulatory responsibility or authority. Many major reservoirs in the mid-Atlantic area are managed by the U.S. Army Corps of Engineers. Information can be obtained via their web pages for the Delaware, Potomac and Susquehanna, and the Youghiogheny river basins.
There are many other reservoirs for water supply and power generation that are maintained by municipalities or utilities. Real-time data are not available at this time for those facilities and the owners must be contacted for additional information. These include the Seneca Reservoir in Montgomery County, Maryland, the Patuxent River reservoirs, and the Baltimore water supply reservoirs. Likewise, real-time data are not available on daily water consumption. However historical water-use data are available from USGS.
The American Whitewater Affiliation provides a compilation of web pages and telephone numbers where real-time streamflow and reservoir information can be obtained across the United States.
How are stations selected for real-time data services?
Existing real-time stream-gaging stations have been established to meet the mission requirements of the U.S. Geological Survey and its cooperating agencies, including water resources management, flood warning, and water resources investigations. Most existing stations were established at the specific request of cooperating agencies with funding support from those agencies, but USGS also has a long-term strategic goal of having real-time access to nearly all of its 7000 stream-gaging stations across the United States.
The schedule for adding real-time access to the remainder of the 110 stations in Maryland, Delaware, and the District of Columbia is a function of funding for satellite or telephone connections, and will probably take many years to complete. In a few cases, the cost may be relatively inexpensive where phone lines are readily available, but in other cases it may cost $5,000-10,000 to add an individual station.
Why are updated data available only every 4 hours?
Data from many real-time streamflow gages are relayed to the Maryland-Delaware-D.C. District office in Baltimore through the Geostationary Operational Environmental Satellite (GOES) data-collection system or by telephone. Data are transmitted from each station at intervals of 4 hours and are loaded onto the District computer system. This 4-hour interval is based on a transmission time window available for the satellite and is not controlled by USGS.
Data from other stations is obtained through an automatic telephone dial-up, and the computer is set to call on a 4-hour interval. USGS does control this system, but it is limited by the actual time that it takes to dial and download data.
USGS is investigating alternative systems to provide data on a more frequent basis during floods. However, because most existing real-time streamflow gages are located on larger rivers and the hydrologic response time of these rivers is generally much greater than 4 hours, the current system is adequate for most applications.
Why are data sometimes not available for a particular stream-gaging station?
Occasionally, a piece of equipment may malfunction or there may be physical problems at a station. USGS tries to correct a station or equipment problem within several days of its first occurrence, and is generally successful in meeting this goal. Possible conditions leading to problems include:
Why do some real-time stations experience equipment problems for extended periods of time?
USGS tries to correct a station or equipment problem within several days of its first occurence, and is generally successful in meeting this goal. Occasionally, replacement parts or equipment may not be readily available, or a station may be inaccessible due to weather conditions.
Most USGS stream-gaging stations are operated in cooperation with other agencies. At some stations, the stage transmitting equipment is owned and maintained by other agencies to support their particular public missions and they may be limited in personnel, parts, or funds to maintain the equipment all of the time.
Data Accuracy and Quality
Why might USGS streamflow data reports not be accurate?
Real-time streamflow data available on USGS pages are PROVISIONAL data that have not been reviewed or edited. These data may be subject to significant change and are not citable until reviewed and approved by the U.S. Geological Survey. Real-time streamflow data may be changed after review because the stage-discharge relationship may have been affected by:
- backwater from ice or debris such as log jams
- algal and aquatic growth in the stream
- sediment movement
- malfunction of recording equipment
Data are reviewed periodically to ensure accuracy. Each station record is considered PROVISIONAL until the data are published. The data are usually published within 6 months of the end of the water year.
Data users are cautioned to consider carefully the provisional nature of the information before using it for decisions that concern personal or public safety or the conduct of business that involves substantial monetary or operational consequences.
Why are there sometimes differences between river stages as reported by the National Weather Service and by USGS?
At some USGS stream-gage installations, NWS maintains a separate stage sensor that is serviced by NWS technicians. Occasionally, calibration of any sensing device may drift from a "true" value, so there may be differences between USGS and NWS data reports. USGS personnel visit installations on an interval of 6 weeks or less to maintain equipment and make required adjustments. NWS technicians have a separate maintenance schedule. In either case, data retrieved from remote sensors is always considered provisional and subject to revision after quality control and analysis. Differences between remotely reported and field-measured data are usually small and within tolerances required for water resources management and control.
I frequently kayak the Potomac River and the real-time stage data reported by USGS seem to be too high (or too low). Are the USGS data inaccurate?
There may be occasional equipment or database problems where erroneous data are reported for short periods of time until corrections can be made. This is why it is important to look at a record of streamflow such as the 7-day hydrograph plots rather than a single point in time. However, most of the time USGS has a high level of confidence in its real-time stage data.
During low streamflow conditions, aquatic grasses may produce increases in stream water level near gages. On smaller streams, debris or rocks on flow control structures may also produce increases in water level. Stage values reported on these pages are believed to be reasonably accurate, but the higher stage readings may produce estimates of discharge that are higher than actual. These higher stage readings at stream gages may be localized and may not be good indicators of stream stage at other locations on the river.
During extreme cold weather, ice can affect stage and discharge determinations at some stream-gaging stations. Data values reported by USGS may be significantly higher or lower than actual streamflow. Adjustment of data for ice effects can only be done after detailed analysis.
Why is the discharge reported at a particular stage sometimes different in different years or at certain times in the same year?
A relationship is developed by USGS hydrographers between stage (usually expressed as feet) and discharge (usually expressed as cubic feet per second). This relationship, often called a rating curve, is developed by making frequent direct discharge measurements at stream-gaging stations. The rating curve depends on the hydraulic characteristics of the stream channel and floodplain, and will vary over time at almost every station. There may be subtle changes to a stream channel, such as the growth of aquatic vegetation in the summer; frequent shifting of a sand-bed stream bottom; catastrophic changes due to floods; or man-made changes such as construction of a bridge. These changes may require only minor or temporary adjustments to streamflow records, or may require a complete reevaluation of the rating curve.
References
Wahl, K.L., Thomas, W.O., Jr., and Hirsch, R.M., 1995 The stream-gaging program of the U.S. Geological Survey: U.S. Geological Survey Circular 1123, 22 p.
Modified from Robinson, Hazell, and Young, 1998
