"ChuckB" wrote

It is my understanding that the level (feet) as measured by a USGS
gauge is not standardized (across many gauges). So the level on one
gauge on a river many not have anything to do with the level on
another gauge on the same river or a different river.

That's correct- the level is based on a somewhat arbitrary datum, thus "5
feet on the gauge" just means that the level is 5 ft. above the datum. The
datum is tied to sea level, so you might have a datum of 0 at 800 ft. above
sea level, so when the gage is at 5 ft., the water surface elevation is at
805 ft. MSL. 0 ft. does not equate to the stream bed either, in many (if
not most) cases.