Stream stage and streamflow

Stream stage

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

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 at Atlanta (02336300) 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.

Rating curve defines stage/streamflow relation

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.

Relation of stream stage to streamflow is always changing

Using Peachtree Creek in Atlanta, Ga. as an example, in July 2003 a stage of 6 feet 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:

• Slope of the stream (affects velocity)
• Roughness of the channel
• Area of the channel at each stream stage
• Backwater effects (when a tributary enters a larger river)
• Filling in, scouring out, channel changes of river banks

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.

For more information:
How does the USGS collect streamflow data?

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