USGS

WHAT IS STREAM GAGING?

Measuring Discharge

Velocity
(feet/second)


Velocity is measured using a meter which turns with the flow of the stream or river. Revolutions of the meter are counted and timed by the hydrologic technician. Velocity measurements done at each subsection represent the velocity for one small area of the stream.

X

Area
(feet x feet or ft2)

Depth and width are measured and areas are calculated by multiplying width (ft) by depth (ft) for each subsection. Width and depth of the subsection are indicated on the diagram.

=

Discharge
(ft3/sec or cfs)


Discharge is calculated for each subsection by multiplying velocity by area. The discharges, in cubic feet per second (cfs), for each subsection are added together to equal the total discharge for a stream or river at the time of measurement.
Diagram showing river cross section being gaged

  • Depth is measured at each velocity measuring position. Velocity measuring positions are selected by the hydrologic technician based on several factors. Usually a stream or river will be measured at about 25 regularly spaced locations across the river.

  • In the gage house there is equipment which records gage height (level of water surface) every 15-minutes and stores this data. Other data such as water temperature and pH can also be recorded. For some sites this data is transmitted to our office via satellite so that we can watch water level changes as they are happening.


Developing a Rating Curve
Example of a rating curve

  • A rating curve is a graphical representation of the relationship between stage (water level) and discharge for a given stream or river. The rating curve is developed from discharge measurements collected over a period of time and over a range of stages (from low flow to flood stage). Each point on the graph represents one discharge measurement. This rating curve can be used to estimate discharge at a given gage height.

  • A rating curve often changes after a flood when the physical characteristics of a river or stream are drastically changed. The rating curve may also change over a long period of time after gradual changes occur.

The above information is available as a poster in PDF format (647K).

U.S. Department of the Interior, U.S. Geological Survey, Troy, NY, USA
URL http://ny.usgs.gov/pubs/posters/streamgagingposter.html
Earth science questions: Earth Science Information Center
Page contact: cleamond@usgs.gov
USGS Privacy Statement
Last modified: 23 Feb 2000