The rating curve is usually plotted as discharge on x-axis versus stage (surface elevation) on y-axis.
Stage is measured by reading a gauge installed in the river.
If the stage-discharge relationship does not change with time, it is called permanent control.
Shifting control is usually due to erosion or deposition of sediment at the stage measurement site.
Bedrock-bottomed parts of rivers or concrete/metal weirs or structures are often, though not always, permanent controls.
If G represents stage for discharge Q, then the relationship between G and Q can possibly be approximated with an equation: where
When a stream is flowing under "channel control" conditions, the parameter