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Missouri River Streamflow-Gaging Station

Bismarck, North Dakota

How Does the USGS Collect Streamflow Data?

Continued from Previous Page

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.

Current MeterVelocity 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

Rating CurveA 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.

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