Gaging Stations & Rating Curves

Gaging stations record stream water level over time. Many people are also interested in the variations in stream flow or discharge over time.

How can you obtain stream discharge from records of stream stage?

At gaging stations, stream discharge can be measured across the stream channel with a flow meter. Numerous measurements of stream discharge are made over a range of stream stages. Remember that stream discharge, Q, is equal to the water velocity multiplied by the cross-sectional flow area. As stream stage falls, the cross-sectional flow area will decrease.

The stream discharge, Q, will decrease as the stream stage and cross-sectional flow area decrease. Increasing stream stage coincides with an increasing stream discharge or flow. The relationship for stream stage vs. stream discharge is unique for a gaging station. A graph of measured stream stages and discharges at the gaging station defines this relationship.

A curve that best represents the position of the data points is called a best-fit curve. This curve can be represented with an equation which can be used to calculate stream discharge from the stream stage data provided by the gaging station. The best-fit curve for stream discharge vs. stage is the stream's rating curve.