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How is a rating curve used to convert gage height into streamflow?

In order to convert gage height (or "stage", usually expressed as feet) into streamflow (or "discharge", usually expressed as cubic feet per second), USGS hydrographers must establish a relationship between them. This stage-discharge relationship is called a rating curve. It’s developed by making frequent manual discrete discharge measurements at stream gaging stations.

The rating curve depends on the physical characteristics of the stream channel and floodplain, and will vary over time at almost every station. There are often subtle changes to a natural stream channel, such as the growth of aquatic vegetation in the summer, frequent shifting of a sand-bed stream bottom, catastrophic changes due to floods, or man-made changes such as construction of a bridge. These changes might require only minor or temporary adjustments to the rating curve, or they could require the development of a new one.

Provisional rating curves for many USGS streamgages are posted in WaterWatch. Place your cursor over the site of interest and left-click. That generates a pop-up window with a rating tab on the far right. Or if you know the site number for a USGS streamgage, plug it into the Customized Rating Curve Builder. For more information about site-specific rating curves, contact the USGS Water Science Center that manages the site.

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