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) Stream discharge
Stream stage: Maybe during a heavy rain you've heard on the radio that "Soandso Creek is flooding and is expected to crest at 21 feet." The "21 feet" is the stream stage, or height in feet above an arbitrarily-assigned level.Since stream discharge normally goes up and down as stream stage goes up and down, the goal is to take enough discharge measurements at enough stages to be able to determine the stream discharge at any stream stage. So, by taking discharge measurements at a river stage of 5, 6, 7, 9, and 10 feet, we should be able to estimate the stream discharge at a stage of 8 feet.Stream discharge: This is the amount of water flowing in a stream at a particular stage, usually expressed as how many cubic feet of water are flowing by each second. Normally, the higher the stage the higher the discharge. A discharge measurement is performed to calculate the amount of water flowing at any given stream stage. For example, at a stage of 21 feet, Soandso Creek might have a stream discharge of 8,000 cubic feet per second (cfs) One cubic foot of water flowing each second is equal to about 7.48 gallons.
Look at the chart below which shows how as stream stage, as measured by gage height on the X (horizontal) axis, increases, streamflow, on the Y (vertical) axis goes up, and it goes up at an ever-increasing rate! The red dots show where an actual streamflow measurement has been taken. We can estimate streamflows at points in between the red dots by drawing a line and reading across to the Y-axis scale. For example, the red dot for 8 feet is about 1,200 cubic feet per second (cfs), and the red dot for 9 feet is about 2,400 cfs. This allows us to estimate that if the gage height was 8.5 feet then the streamflow would be about 1,700 cfs. You can see this by looking along the X axis (gage height) to 8.5, draw an imaginary line upward until you hit the curving line, and then draw an imaginary line to the left until you hit the Y axis (streamflow). Your imaginary line should hit the Y axis at about 1,700.
Notice how the curve gets steeper as the gage height gets higher. This means that streamflow increases at a faster rate as the gage height goes up. For example, when the gage height goes from 6 to 7 feet, the streamflow increases from about 400 to 600 cfs, about a 50-percent increase. But, when the gage height goes from 9 feet to 10 feet, the streamflow increases from about 2,400 to about 10,000 cfs -- over a 400-percent rise! The reason for this is that river banks tend to flatten out as they get higher in height.
This means that it is important to get a discharge measurement when it rains a lot, and the gage height is high -- it helps to define the valuable information contained in the high end of the curve (we need the red dots at the high gage heights). That is why when it storms, day or night, hydrologists rush out to measure streamflows.
Information provided by: http://wwwga.usgs.gov