River Gauge Zero?

Does anyone have any idea how the level of “zero” was selected when the USGS river gauges were first installed?

Was it totally arbitrary for each gauge?

No it is not arbitrary.
At some point during the installation of a gauge, a survey was conducted and a marker is placed nearby. That marker contains the exact elevation above sea level along with the degrees North and East. This reference point is usually set close, but not perfect to the average value of the river. It just needs to be a reference.

The river fluctuates up and down from the reference mark. There is really no relation to the gauge reading verses the actual depth of the water at the gauge house. By that I mean, the gauge can say 2 feet, and the river can be bone dry in front of the gauge house. Or the river might be 20 feet deep in front of the gauge house for the same 2 feet gauge reading.

One gauge on the Sinnemahoning near hear, often reads - 1.5 feet, yet there is still water in the river. So, it is just a deviation from a known reference that is located near the gauge house specifically to serve as that function.

Then, you have to check the gauge and observe your river of interest to see how that number specifically relates to water depth for the area you wish to paddle. The Allegheney gauge at Eldred, when it shows less than 3 feet, is not floatable in many sections. Certainly, any canoe will float in 3 feet of water. But there is only 3 inches of water in the muddy flats when the gauge reads 3 feet.

So, in one sense, I guess you could say it is arbitrary, as five gauges can all read 2 feet and the water depth at each location may vary considerably. But the method of surveying and locating a known reference marker is a well defined and regulated process.

great question and answer.

I knew the guage readings were all indexed to some independent variable, other than water level, but I never knew what it was.

The day is young and I already learned something. And, there’s a boat on my vehicle that will get wet today. It is shaping up to be a great day.