How streamflow is determined

Measuring streamflow

In order to accurately determine streamflow, measurements must be made of its width, depth, and speed (velocity) of the water at many horizontal and vertical points across the stream. To develop a stream-stage/streamflow relation (rating curve), streamflow must be measured at many different stages. The well-developed rating curve allows for estimation of streamflows at virtually any stream stage. More simply, if a stream is measured at stages of 3.5, 6, 7.1, 9, and 10.2 feet, then an estimate can be made for a streamflow at 8 feet -- that is the goal.

For example, let's say we need a measurement of Example Creek when it is at a stream stage of approximately 3 feet. First, someone has to go out to the stream when the stage is near 3 feet. The diagram below shows a cross-section of Example Creek at a 5-foot stage. Note that the stream stage does not necessarily correlate to the actual depth of the stream. Example Creek is about 10 feet wide. The stream-measurement procedure is to go across the stream at selected intervals and measure the total depth and the velocity of the water at selected depths at each interval across the stream. The picture shows a current meter (attached above the torpedo-looking weight), which is lowered into the stream and measures water velocity. The spinning cups on the current meter measure velocity.

In the diagram, the hydrologist would take a measurement of how fast the water is moving at every green 'X', and would then determine the areas between all of the measured intervals, such as the one shown by the purple box.

In the diagram, water depth/velocity measurements are obtained horizontally across the stream at 1, 3, 5, 7, and 9 feet (the vertical lines in the diagram). At each location, measurements of velocity and total depth are obtained. Depending on the depth and flow conditions, one or more velocity reading(s) are obtained in each vertical. For our example, a water depth/velocity measurement is obtained at a point 5 feet from the edge of the stream. The total depth is slightly more than 3 feet and velocity readings are obtained at depths of 1, 2, and 3 feet (the 'X's on the 5-foot vertical line). The purple box represents an area that is midway between this measurement point and the measurement points on either side. The purple area is 2 feet across and one foot high, or 2 square feet. The measured velocity at the big X in the purple box is is 2 feet per second. To compute the amount of water flowing in that purple area each second, multiply the area of the purple box times the velocity of the water:

(1) 2 feet wide x 1 foot high = 2 square feet
(2) 2 square feet x 2 feet per second = 4 cubic feet per second.

To compute the total stream streamflow the hydrologist has to create imaginary purple boxes between all of the 'X's and, using the velocity of the water in every box, compute the streamflow for each purple area. Summing the streamflows for all the purple areas will give the total streamflow. Actually, the example above is a simplified explanation of how streamflow is measured. When an actual measurement is made, the hydrologist takes measurements at about 20 points across the stream. The goal is to have no one vertical cross-section contain more than 5 percent of the total stream discharge.

For more information:
How Does the USGS Collect Streamflow Data?

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