Large-scale turbulence in the river causes velocity pulsations on the order of 20 seconds to several minutes. Long-term velocity pulsations are also induced by bedform migration. Fortunately, the averaging of velocity measured for short durations at numerous cross sections reduces the error in the mean velocity for the stream. Changes in the measurement cross section are understood to affect the rating by showing more or less discharge for a given river stage. Compensating adjustments are applied to the stage record to account for changes in the cross section so that discharges remain accurate.
Uncertainty remains, however, during storm flows when the stream scours and fills unpredictably.
The changes in stage do not necessarily follow the direction of scour and fill, as shown at Stage changes during streambed fill and scour.