Stage is generally measured continuously and reported to an accuracy of 0.01 foot. Stage information is transmitted by satellite radio to USGS computers, or is stored on site and retrieved periodically by USGS staff.
Flow is more difficult to measure accurately and continuously than is stage.
Individual discharge measurements are made at the gaging station using standard procedures, ideally during a time when the river stage is not changing. A series of these measurements made over a range of flow conditions defines the rating curve, which is used to convert continuous measurements of stage to a continuous record of flow. Channel changes, resulting from scour, deposition, or other processes, alter the stage-discharge relation, so that discharge measurements must be made routinely to ensure that the rating curve remains accurate. The rating curve is considered accurate only over the range for which discharge measurements have been made. Discharge measurements generally are not available for the full range of flows at gaging stations which have been in operation for only a few years.