Stage-discharge relations (ratings) are usually developed from a graphical analysis of numerous current-meter discharge measurements. 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 bed or channel vegetation. Such changes are called shifts; they may indicate a short- or long-term change in the rating for the gage. In normal useage, the measured shifts (or corrections) are applied mathematically to a defined rating. If a given shift is found to be stable over time, a new stage-discharge rating is developed and the shift removed.
The tables being provided are shift corrected, incorporating the mathematical adjustments for ease of use by the recreational user. 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 monthly, others may not change for years.