When they say, “river crested at 26 feet” where are they measuring from? The bottom of the river? The ‘normal’ water level?
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from “normal” river levels; at the time the measurement tool was placed. this might mean we strapped a gage to the bottom of a bridge or near some easy to access area.
over time rivers might raise or lower naturally and the gage isnt always ajusted; its possible for the “norm” to be at -10inches or +18inches
Around here floods get quoted as one of two things:
One is with respect to “flood stage”. Which is a level defined as “things are about to cause damage or disruption” and then everything is reported as water levels above that. This is nice because it’s fairly easy for people to understand which is nice for say a TV news report, but sometimes can cause confusion if it varies a lot based on where exactly you are on a body of water.
The other is gauge zero. The gauge is a device that measures water levels in the river. Often times “Zero” here is something that was decided on a long time ago based on something that made sense at the time. There is usually a lot of opposition to changing where zero is set because it’s a potential source of confusion if you’re looking at historic data.
Every once in a while you also get readings from sea level. I’ve never seen one locally but I’ve heard they’re more common out in the Western US.
What happened to “explain like I’m 5”?
The people measuring will have a set datum. In my city, which has pretty regular floods in the spring, the datum is set as the historic average winter ice level at the city’s old pumping station (which was on James Avenue, so the City refers to river height as “4.5 feet James”.
Measuring rivers in feet can get weird, as the top commenter said. For example, the New River in WV regularly goes below 0 feet, but that doesn’t mean it’s empty, just means it’s running below the level someone decided was “0 feet.” I believe at 0 feet, the New River is around 2,400 cubic feet per second (cfs), which is a more accurate way of describing level. The New is at flood stage once it’s over 12 feet, which is somewhere around 33,000 cfs. There is a rock down at the beach that is painted with the foot gauge, but you really have to see the individual river’s physical gauge to get an idea of what 26 feet means for that river. Looking up the cfs could give a better idea of the actual volume of water.
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