• What is a Rating?
Stage-discharge relations (ratings) are usually developed from a graphical analysis of numerous current-meter discharge measurements (sometimes called calibrations). Measurements are made on various schedules and sometimes for different purposes. All discharge measurements are compiled and maintained in a data base. Each measurement is carefully made, and undergoes quality assurance review. Some measurements indicate a change in the rating, often due to a change in the channel or riparian vegetation. Such changes are called shifts; they may indicate a short- or long-term change in the rating for the gage. Applying these shifts to a rating is called a Shift-Adjusted Rating.
• What is a Shift-Adjusted Rating?
The stage-discharge relation in a natural channel is seldom static because the physical features of the channel often change either gradually or abruptly. These changes are referred to as "shifts" to the base rating.
Shifts are either positive or negative, depending upon whether or not it's value must be added to or subtracted from the recorded gage height to adjust for the departure from the base rating. Sometimes, negative shifts are referred to as shifts to the left and positive shifts as shifts to the right. Possible causes for negative shifts include fill or deposition in the channel, temporary dams (natural or human-made), seasonal vegetative or algal growth, and debris jams. Possible causes for positive shifts include scour, gravel mining, and clearing of debris or vegetation from channel either by floods or humans.
In normal usage, the measured shifts (or corrections) are applied mathematically to a defined rating. The shift adjustments are applied to the individual ratings as measured data becomes available, resulting in an adjusted rating. Some ratings may change as often as weekly, others may not change for months.