USGS Stage-Discharge Relation Example

USGS Stage-Discharge Relation Example.

The continuous record of stage is converted to streamflow by applying a mathematical rating curve. A rating curve (fig. 3) is a graphic representation of the relation between stage and streamflow for a given river or stream. USGS computers use these site-specific rating curves to convert the water-level data into information about the flow of the river.

The rating curve is developed from a number of physical discharge measurements collected over a period of time and over a range of stages (from low flow to flood stage). Each point on the graph represents one discharge measurement. Streamflow is measured in cubic feet per second, which is equivalent to a box of water measuring 1 by 1 foot with a flow rate of about 7.5 gallons per second. The stage-discharge relation depends on the shape, size, slope, and roughness of the channel at each gage and is different for every streamgage. A rating curve often changes after a flood when the physical force of high water movement can change the dimensions of the streambed or stream channel. To keep the rating curve accurate and up-to-date, USGS hydrologic technicians visit each streamgage about once every 6 weeks to measure the flow directly and also measure high flows when they happen.