If you're a teenager, I imagine your favorite activity is to sit with your parents on a quiet river bank, drink your glass of lemonade, and ponder the complexities of life. Probably the first question you ask is "How much water is flowing in this river?"
You've come to the right place for an answer. The U.S. Geological Survey has been measuring streamflow on thousands of rivers and streams for many decades. It is a process involving two concepts:
(1) Stream stage
(2) Streamflow
Often during a large rainstorm you can hear an announcement on the radio like "Peachtree Creek is expected to crest later today at 14.5 feet." The 14.5 feet the announcer is referring to is the stream stage. Stream stage (also called stage or gage height) is the height of the water surface, in feet, above an established datum plane where the stage is zero. The zero level is arbitrary, but is often close to the streambed. You can get an idea of what stream stage is by looking at this picture of a common staff gage, which is used to make a visual reading of stream stage. The gage is marked in 1/100th and 1/10th foot intervals.
Streamflow, or discharge, is the volume of water flowing past a fixed point in a fixed unit of time. For water flow in streams, the U.S. Geological Survey expresses the value in cubic feet per second (ft3/s). For example, when rain has not fallen for a while, Peachtree Creek often is at a baseflow stage of about 3 feet. The rating curve (see chart below) shows that at a stage of 3 feet streamflow is 76 ft3/s. Since one cubic foot of water contains 7.48 gallons, it might be easier to understand this streamflow value if you consider that 76 ft3/s is about 568 gallons of water flowing each second.
This chart, known as a rating curve, shows that there is a relation between stream stage and streamflow. The stage-streamflow relation is used to relate water level to an associated streamflow. The rating curve for a specific stream location is developed by making successive steamflow measurements at many different stream stages to define and maintain a stage-streamflow relation. These steamflow measurements and their corresponding stages are then plotted on a graph. Continuous streamflow throughout the year can be determined from the rating curve and the record of river stage.
The rating curve is crucial because it allows the use of stream stage, which is usually easily determined, to estimate the corresponding streamflow at virtually any stream stage.
In July 2003, a stage of 6 feet at Peachtree Creek translated into a streamflow of 727 ft3/s, but that number could change by July 2004. Rating curves are not static - they occassionally must be recalculated. Rating curves frequently shift due to changes in the factors that determine the relation between stream stage and streamflow. These factors are:
Consider what can happen to a stream channel during a large flood. The diagram below shows a streambed before and after a flood, thus changing the relation between the stream stage, in feet, and the amount of water flowing at that stage. The colored area represents how much water is flowing. Both diagrams show the same 5-foot stage, but more water is flowing after the flood because the streambed profile has changed and now there is more area for water to flow. Scouring occurs more often on the outside edge of a curve in a stream, whereas sand buildup occurs on the inside edge.
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 3-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.
Read a
personal account of measuring flow during a flood in California.
Sources and more information
• Discharge measurements at gaging stations, USGS Techniques manual
• A Day in the Life of a USGS Water Scientist
• Willie Takes a Field Trip, a coloring book that introduces tomorrow's scientists to water
The water cycle: Streamflow
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/measureflow.html
Page Contact Information: Howard Perlman
Page Last Modified: Wednesday, 13-Aug-2008 07:20:49 EDT
