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Measuring Streamflow

Accurately measuring streamflow at each visit to the site is critical to streamgaging. The correct equipment for each stream during different seasons ensures the highest quality data are collected each time.

The WY-MT office of the USGS measures streamflows (discharge) that range from a few gallons per minute to several hundreds of thousands of cubic feet per second.


flume measurement



When measuring very small flows, oftentimes a portable flume is used to "gather" all the water through a confined throat. The geometry of the flume enables one to use a chart to know how much flow is moving through based on the height of the water in the flume. This measurement was taken on Dugout Creek in Montana.






Diagram of Channel Cross Section With Subsections



Most streamflow (discharge) measurements use a technique where the cross-section of the stream is "cut" into subsections. Depth, width, and velocity are measured in each section to calculate the volume of streamflow (discharge). The streamflows in all sections are then added up to get the total for the stream.

This general principle is used whether measuring the streamflow with a handheld current meter or a meter that is attached to a small boat for remote deployment.







Streamflow measurement in S Fk Spread Creek, WY (13012475)



When making a streamflow measurement while wading a stream, the hydrographer determines the depth using the wading rod that is etched with a “ruler”, the width using the tag line strung across the stream, and the velocity of each subsection using a current meter.





Measuring streamflow on St.Regis River at St Regis




The current profiler uses SONAR to measure not only velocity, but also depth and width. The hydrographer pulls the boat across the cross section to allow the profiler to measure stream parameters and calculate the streamflow.





Get the whole picture: From Gage to Page: A Look Into How USGS Helps You Know the Flow

The steps involved in getting the data to your fingertips, generally include:

  1. Establishing a streamgage
  2. Measuring the streamflow (explained above)
  3. Creating a stage-discharge relation
  4. Transmitting data via satellites to computer systems and onto the web