A Stream's Rating Curve

As mentioned on prior pages, the variable stream velocity across a stream and irregular channel shapes make the task of estimating stream discharge difficult. However, it is possible to estimate a stream's discharge by knowing the height of the water in the stream. Over a period of years, scientists keep track of stream height and discharge at "GAGING STATIONS " and from this information they construct a "RATING CURVE." (To look at some illustrations of stream gage stations, click HERE.)

To illustrate the process of building a rating curve, let's do a simple comparison of a stream's discharge and the level of water in the stream. This level is the stream's STAGE, which is defined as the height of the stream above its bottom (or some other reference or "datum" level.

Below is a graph and a table of Stage and Discharge data from a Gage House along the Gunnison River, in central Colorado. The data are annual peak flood values from 1945 to 1998.

Your task is to make a graph from this data, which will be the basis of our Rating Curve.

• Use your mouse and drag to the graph each of the red symbols representing measurements of stage and discharge data. Move each point until it's coordinates (stage, discharge) match those from the table. Correctly located points are blue in color.
• Once you have the 4 other points plotted, click the "Add more data" button which will add about 50 more points. (Coordinates will no longer be displayed.)
• Click the "Fit Curve to Data" button. Although beyond the level of this introductory activity, the curve is actually the result of statistical "Least Squares" fit.
• As soon as the "Curve" is displayed, a "NEXT " button will appear below the table. Click it to go to the next page.
• (If you want to return to the previous page, use your browser's "back" button.)