Gauge height
What does a gauge height reading mean?
A gauge height is how high the water surface stands above a fixed zero point at the gauge, in centimetres, metres or feet. It is a height above that point, not a depth.
On this page
What a reading is
A river gauge measures the height of the water surface above a zero point that belongs to that gauge. The agency that runs it gives the reading in centimetres, metres or feet. The US Geological Survey calls the same thing gage height and defines it as the height of the water surface above the gage datum, the reference point of the site.
Agencies also call it stage, river stage, river level or water height. A reading of zero does not mean the channel is dry, and a reading below zero is possible: zero is only the zero point of that gauge.
Where the zero point comes from
The agency that runs a gauge chooses its zero point. The US Geological Survey says each site has a reference point chosen for that site and generally placed below the streambed, because streambeds change over time. Near the ocean it may be set at sea level.
PEGELONLINE, the online service of the German waterways and shipping administration, says its readings are relative to the zero point of each gauge. It also publishes where that zero lies on the national height scale, and our place pages show it, for example as metres above NHN, the German sea-level datum.
Two gauges with the same reading therefore do not have the same water height above the sea, unless their zero points happen to lie at the same height.
For many lakes and reservoirs the agency gives the height of the water above sea level or another named datum instead of a gauge zero. The place page says which, for example "above mean sea level".
Why it changes with the channel and the datum
The channel changes. The US Geological Survey notes that streams erode and become deeper, or deposit sediment and become shallower. The zero point is chosen so that it does not move when the bed does, so the same reading can lie over more or less water after the channel has changed.
The datum can change. A datum is the agreed base that heights are counted from. Agencies change a gauge's zero point from time to time, and PEGELONLINE publishes the date from which each gauge's current zero is valid. A new valid-from date can mean the zero point was moved, and then readings from before and after are not on the same scale. It can also mean only that the height of the same zero was measured again on a new height system, and then the readings do not change.
Datums also differ between agencies. Our pages name the one the source gives, for example NAP for the Dutch readings and the North American Vertical Datum for many United States ones.
What this means when you read a gauge
Compare a reading with the same gauge's own past, not with another gauge. That is how this site works: it compares each gauge with the normal range for the same date at that same gauge.
A reading says how high the water stands against that gauge's zero. It does not say how deep the river is, and it does not say that a flood is near.
Where this page comes from
The definitions are the agencies' own. Their pages are named below, not linked. Search for each name to reach its page.
- US Geological Survey, Why we use gage height
- PEGELONLINE help (German waterways and shipping administration)
Changes
- : First published.