The rating curve is a relation between stage (river level) and streamflow (discharge). Each stream channel is different and, because the stage-discharge relation is a function of the streambed material and geometry, each rating curve will be unique to that site and a particular period of time.
Rating curves are developed over time. Each time a site is visted, stage and discharge are recorded. After sufficient a sufficient number of stage-discharge pairs are collected and plotted, data are collected, a line (curve) is drawn through the points to describe the relation.
As additional data are collected, the rating curve can be refined to more accurately reflect the relation. As is noticeable in the graph to the right, the addition of stage and discharge measurements at high and lower flows change the slope of the line (the rating curve) and create a more accurate relation at a larger range of stages. This more accurate relation is important, because as can be seen on the graph, the selected stage would have a much different associated discharge with the original rating curve (the orange vertical line) compared to the revised rating curve with additional data showing the discharge is lower (green vertical line).
Get the whole picture: From Gage to Page: A Look Into How USGS Helps You Know the Flow
The steps involved in getting the data to your fingertips, generally include:
- Establishing a streamgage
- Measuring the streamflow (discharge)
- Creating a stage-discharge relation (explained above)
- Transmitting data via satellites to computer systems and onto the web