Mar
19
2010

Measuring the height of a flood

The Mississippi River here in Saint Paul is currently forecasted to crest at 19.8 feet next Wednesday and was measured at 12.9 feet at 7:00 this morning (Friday). All these river height numbers got me thinking about a really good question someone asked me last year about just what these numbers mean and how they are measured.

Stream gauge location: Readings of the Mississippi River at downtown St. Paul are taken by instruments here.
Stream gauge location: Readings of the Mississippi River at downtown St. Paul are taken by instruments here.Courtesy USGS

Here is an explanation: The Mississippi here in Saint Paul started being measured way back in 1893. The actual location of the measurement station is right by the High Bridge on the west side of the river. Currently the station is operated by the US Geological Survey and the US Army Corps of Engineers. At the time that the station was established, an arbitrary 0 measuring point was chosen (probably the bottom of the channel at the time). When the river reaches 14 feet at this station it is pretty much flowing above its banks in the vicinity of the gauge. All this is to say that the gauge numbers aren't really measuring anything specific about the river other than its height above a point established over a 100 years ago. This means that gauge readings can only be compared to other readings at the same gauge.

Kate's photos, 3/18 (2): Look across the river to the floodwall: thats the high-water mark for the 1965 flood, the highest in recorded history. That year, the river crested here in downtown St. Paul at 26.01' and spelled the end for the communities down on the river flats.
Kate's photos, 3/18 (2): Look across the river to the floodwall: thats the high-water mark for the 1965 flood, the highest in recorded history. That year, the river crested here in downtown St. Paul at 26.01' and spelled the end for the communities down on the river flats.Courtesy Kate Hintz

So if you call up your friends and family in Fargo/Moorhead today to compare notes about who is experiencing the worst flooding, you'll find that your Mississippi River measurement of 12.9 feet doesn’t look impressive by the measurement of over 35 feet at Fargo. And, your 12.9 feet will seem straight up puny compared to the 677 feet the Mississippi is flowing today at Prairie Island, Minnesota, where the river is measured against elevation.

For details on just how river gauges work, check out this explanation the US Geological Society offers.

Noteworthy flood heights in Saint Paul:
14.0 Portions of the Lilydale park area begin to experience flooding.
17.5 Harriet Island begins to become submerged.
18.0 Warner Road may become impassable due to high water.
19.8 Forecasted crest
26.4 Record 1965 crest!

Want to learn more about floods and the Mississippi River? Stop by the Mississippi River Visitor Center in the lobby of the Science Museum and talk to a National Park Ranger!

Check out our full feature on the 2010 Mississippi River flooding.

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