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How/Why Does the USGS Collect Streamflow Data

Learn how and why the USGS collects streamflow data.

How Does the USGS Collect Streamflow Data?

1.  A Gage Site is Established

The U.S. Geological Survey selects a suitable site along a river or stream and constructs a gage house (pictured at left) to hold equipment that measures and records the height of the water surface (gage height or stage). The gage house also can hold equipment that measures water-quality parameters, such as temperature, pH, dissolved oxygen, and dissolved chemicals, and weather conditions, such as air temperature, precipitation, and wind speed.  









2. The Water Level is Measured and Recorded

The gage height (or stage) of a river most commonly is measured through the use of a stilling well, a bubble system, or a wire-weight gage.

  • stilling well (pictured at right) is used when a gage house can be built immediately adjacent to a river.  The well is connected to the stream with pipes so that when the water level in the stream changes, the water level in the well also changes.  A float in the well is connected to a recorder or data-collection platform. The Bismarck gage house uses a stilling well to measure gage height.




  • The bubble system (pictured at left) can be used when construction of a well is not feasible.  The bubble system requires a long open-ended pipe that extends from the gage house to the river.  One end of the pipe is fixed securely below the water surface, and pressurized gas (usually nitrogen or air) is forced through the pipe from inside the gage house and out a submerged opening called an orifice.  The pressure in the pipe is determined by how deep the water is over the orifice.  A change in the water level of the river produces a corresponding change in the pressure in the pipe.  The change in pressure is converted to an electronic signal by a transducer inside the gage house.  Data from the transducer then is fed to a recorder, or data-collection platform, which records the corresponding gage height. An outside reference gage, typically a vertical graduated ruler called a staff gage, is read periodically to verify that the recorded gage heights from the stilling well or bubble system are the same as the water levels in the stream.



Cantilever Gage
  • wire-weight gage (pictured at right) consists of a drum wound with a single layer of stainless-steel cable attached to a bronze weight, a graduated disc, and a counter, all within a cast-aluminum box.  The weight may be lowered to the water surface using a hand crank.  When the bottom of the weight is at the water surface, the gage height is indicated by the combined readings of the counter and the graduated disc.  The wire-weight gage commonly is mounted on a bridge handrail, parapet wall, or pier for use as an outside gage.




3. Discharge and Velocity are Measured

The volume of water passing a specific point in a given interval of time is called discharge.  Discharge generally is measured in cubic feet per second and most often is determined by making measurements of a particular cross-section area of the river and of how fast the water is flowing per second (velocity) past that cross section.  Discharge then is calculated by multiplying the width, depth, and velocity of that section of the river.

Velocity is measured by using a current meter (pictured at right).  The meter consists of cups that are rotated by the action of flowing water.  The speed of the rotation depends on the velocity of the water passing by the cups.  A technician counts the number of revolutions in a given time interval and determines the velocity at the location of the meter.  The stream is divided into segments and the average velocity and depth of the stream are measured at each segment.  The discharges in each of the segments then are summed to obtain the total stream discharge.  Technicians obtain measurements in several ways, depending on the size and depth of the river.  They will wade across small streams, use suspended cableway cars, stand on bridges, or work out of boats to measure discharge in large rivers.







4.  A Relation Between Water Level and Discharge is Developed

A stage-discharge relation or rating curve is used to relate water level to associated discharge.  The rating curve for a specific stream location is developed by making successive discharge measurements to define and maintain a stage-discharge relation.  These discharge measurements and their corresponding stages (or gage height) then are plotted on a graph for each station.  Continuous discharge throughout the year can be determined from the rating curve and the record of river stage (gage height).  Factors such as ice cover, debris, and vegetation growth can affect the stage-discharge relation, and the data must be checked periodically to ensure accuracy.




Real-Time Streamflow Data Available on the World Wide Web

Telemetered systems are used when current or real-time information on stream-gage height, discharge, or weather or water-quality parameters is needed.  Real-time gage-height information is necessary for maintaining reservoir levels, forecasting floods, and other purposes.  The most common telemetered system is the satellite data-collection system.  From an antenna attached to the roof of a gage house, electronic signals are transmitted to a satellite orbiting the earth.  The data then are retransmitted to receivers at various locations throughout the country.  These receivers then transmit data to USGS computer systems that make the data available on the Internet.

Real-time data for river and streams throughout the nation are available at:




Why Does the USGS Collect Streamflow Data?

The quantity and quality of water in most of North Dakota's rivers, streams, lakes, and reservoirs is monitored by the U.S. Geological Survey.  The monitoring program is a cooperative effort that is funded jointly by numerous Federal, State, and local agencies.  Streamflow data collected as part of the monitoring program are used for:

  • Designing water-storage and treatment facilities
  • Forecasting floods and droughts
  • Managing and operating reservoirs for flood control, hydropower, and water supply
  • Designing bridges, culverts, and other hydraulic structures that will function safely during floods
  • Maintaining water quality and developing cost-effective plans to improve water quality
  • Hydrologic research 
  • Planning canoeing or kayaking trips to avoid unsafe or unsatisfactory river conditions

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