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Hydrographic Effects

Hydrographic Effects

What causes the odd squiggles in a river stage chart?

At gaging stations, a continuous paper record of river stage can be drawn by a pen moved by a stage-sensing device. Most chart recorders have been replaced by digital recorders of several designs, but older charts are available in USGS archives.

Hydrographers first try to assure that changes in recorded river stage represent only changes in the actual river stage. (Instrumentation problems such as stuck floats, frozen intakes, or loss of nitrogen gas make odd squiggles, too, but their source is usually obvious to the hydrographer.)

If stage changes are real, then the streambed observations show an intermediate effect: the river stage can rise and fall in response to migration of river bedforms. Growth of sand bedforms is well known for sand-bedded streams. In flooded gravel rivers, one source of those squiggles could be small gravel bedforms that migrate past a gaging station and affect the local river stage.

Some examples from gravelly California streams show odd squiggles during flow recession. The question remains: are these squiggles caused by gravel dunes?

Rapid velocity variations

A routine measurement of river velocity for a discharge measurement encompasses about 40 seconds. Examples of velocity variations during observation periods of several minutes are shown at Velocity Observations. 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.

If a discharge measurement doesn't plot on the rating, who's off? You, or the river?

A "rating" refers to the empirical relation between river stage and discharge that is established by standard techniques. 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.

(See the chapter titled "Determining a Continuous Record of Discharge" in the online report Stream Gaging Program of the USGS for a discussion of discharge ratings.)

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Page Last Modified: Wednesday, 05-Oct-2011 15:15:16 EDT