In order to convert water height (or “stage”, usually expressed as feet) into a volume of water (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 direct discharge measurements at stream gaging stations.
The rating curve depends on the hydraulic characteristics of the stream channel and floodplain, and will vary over time at almost every station. There might be subtle changes to a 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 streamflow records, or could require a complete reevaluation of the rating curve.
The USGS Waterwatch Toolkit includes a Customized Rating Curve Builder to generate rating curves for individual gaging sites and a Rating Depot for more detailed data (plug a new site number into the URL). More information about site-specific rating curves is available from the USGS Water Science Center that manages the site.