Discharge and Stage

How River Discharge is related to Stage (Continued)

Below is the Rating Curve that you constructed for the Gunnison River. Although the fit of the curve to the data is not perfect, the graph can be used to estimate discharge knowing only the "stage" value of this stream. Many streams in the US have Rating Curves established for a gage houses alongside the stream. Using the stage data of a stream, a CONTINUOUS RECORD of discharge over time can be developed. We shall look more at how stream discharge changes over time in a few moments.

As you can see on the graph, at high flow values, a small increase in stage corresponds to a very large increase in the discharge of the Gunnison River.

This kind of relationship, where a slight increase in one value (stage) corresponds to a large increase in another quantity (discharge) is an exponential one. It can sometimes be better represented as a "semilog" plot. where the vertical axis is logarithmic and the horizontal is linear. Click the "Semilog Graph" button to change the scale. Most rating curves are presented as seimlog graphs.

Using either the standard curve or the semilog equivalent, answer these questions then click Next. (Vertical spacing of gray lines on the standard graph is 2000 cfs, whereas on the semilog graph, it's 5000 cfs.)

1. What is the approximate stage value when stream discharge is about 4000 cfs?
2 ft 4.5 ft 5 ft 5.6 ft 8 ft.

2. Although the Gunnison River has not been know over the past 50 years to reach a stage of 10 ft, it could happen. What will be the approximate discharge at the 10 ft stage?
5000 cfs 7500 cfs 10000 cfs 15000 cfs