Why is the discharge reported at a particular stage sometimes different in different years or at certain times in the same year?
A relationship is developed by USGS hydrographers between stage (usually expressed as feet) and discharge (usually expressed as cubic feet per second). This relationship, often called a rating curve, is 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 may 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 may require only minor or temporary adjustments to streamflow records, or may require a complete reevaluation of the rating curve.
