A stage-discharge curve is a graph of water surface elevation versus flow rate in a channel. A stage-discharge curve is shown in Figure 7‑12. You may compute various depths of the total discharge for the stream, normal flow channel, and floodplain.

(See and .) The data, plotted in graphic form (sometimes termed a “rating curve”), gives you a visual display of the relationship between water surface elevations and discharges.

An accurate stage-discharge relationship is necessary for channel design to evaluate the interrelationships of flow characteristics and to establish alternatives for width, depth of flow, freeboard, conveyance capacity and type, and required degree of stabilization.

The stage-discharge relationship also enables you to evaluate a range of conditions as opposed to a preselected design flow rate.

Examine the plot of stage-discharge carefully for evidence of the “switchback” characteristic described below. Also, examine the plot to determine whether or not it is realistic. For example, a stream serving a small watershed should reflect reasonable discharge rates for apparent high water elevations.