Water Q&A: Does stage tell you how much water is flowing?

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Learn how river "stage" relates to streamflow and discharge, and how the USGS calculates them.

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Does stage tell you how much water is flowing?

Water Questions and Answers

Yes, but not directly. The U.S. Geological Survey (USGS) has to go through a lot of computations to decipher the relation between river stage and streamflow, which is different at every stream monitoring location. And, you cannot say that because a stream rises (doubles) from a 10-foot stage to a 20-foot stage that the amount of water flowing also doubles. When a river flows out of main river banks and spreads out along the flatter landscape, even a 1-inch rise in stage can mean a huge rise in streamflow. So, the amount of water flowing in a stream might double when the stage rises from 5 to 10 feet of stage, but then it might quadruple when it goes from 15 to 20 feet. This graphic, called a rating curve, helps to illustrate, as it shows a typical river stage-discharge (streamflow) relation.

To find out how much water is flowing in a stream or river, USGS personnel have to go out and make a "discharge measurement"—a measurement of streamflow at that instant in time. USGS uses the term "discharge" to refer to how much water is flowing, and discharge is usually expressed in cubic feet per second. The term "streamflow" can be used instead of discharge. To measure instantaneous discharge, USGS sends hydrologic technicians to go out and stand in the stream, on a bridge, or in a boat, and measure the depth and how fast the water is moving at many places across the stream. By doing this many times and at many stream stages, over the years USGS develops a relation between stream stage and discharge.

USGS Stage-Discharge Relation Example

Example of a typical stage-discharge relation; here the discharge of the river is 40 cubic feet per second when the stage is 3.30 feet. The dots on the curve represent concurrent measurement of stage and discharge.

Stream stages are not always cooperative, so its not uncommon for someone to have to go measure a stream at 2:00 in the morning during a storm, sometimes in freezing conditions. Also, the stream can be uncooperative in that it changes -- a big storm may come along and scour out bottom material of a creek, or lodge a big log sideways in the creek, or sometimes do both at the same time. These kind of changes result in changes in the relation between stage and discharge.