Understanding river stages can leave your head swimming

Q. When looking at your weather page I see that the flood stage of most of the surrounding rivers is 20-30 feet. However, the river stage of the Kaskaskia River at New Athens is usually about 70 with a flood stage of more than 79. Is this any reflection of the depth or is this a typo?

A. What could be simpler than measuring the depth of a body of water, right? Just stick in a long ruler or use sonar and take your reading. That’s how they do it with swimming pools, so why not a river?

In a perfect world, it might be that simple but this is far from a perfect world, says Gary Johnson, chief of the Hydrologic Data Section of the U.S. Geological Survey’s Illinois Water Science Center in Urbana.

Here’s your problem: Let’s take your example of the Kaskaskia “at” New Athens. Where are you going to measure its depth? Even within a short stretch of the river near the town, the depth may vary by several feet because there’s usually no constant slope to the river bed.

“There’s always deep pools and riffles, pools and riffles,” Johnson said. “For example, if you walk along Silver Creek, you might go from a pool 4 or 5 feet deep to a gravel bar 6 inches deep to an even deeper pool 6 to 8 feet deep. That’s why we stay away from (simple depth measurements).”

Instead, decades ago, scientists developed what you’ll likely find is a convoluted way of establishing river stages based on the height of the river surface above sea level. With Johnson’s help, I’ll try to make this somewhat clearer than the mud on the river bottom:

Let’s say you’re going to establish a river stage at Anytown, Ill., on the Mississippi. On the first day, you find that the surface of the river is 416 feet above sea level. So far, so good. But to give that figure any meaning, you have to compare it to something. We’ve already rejected the idea of measuring the river’s exact depth at any given point. So what we do is pick a sea level below which we assume the river will never fall. Let’s say we establish that level to be 400. So from then on, the river stage will be the difference between the actual elevation of the river surface and this hypothetical level that the river will never fall below. On this first day, the river stage at Anytown is 16, although as you can see it may have little to do with the actual depth.

“This (hypothetical elevation) is what we call a ‘datum,’” Johnson said. “The datum is a flat plane at an arbitrary elevation from which everything else is measured. At every gauge, we establish a datum.”

For example, the datum of the Mississippi at Grafton is 403.79. Currently, the river stage is 15, which means the surface of the river is 418.79 feet above sea level. It does not mean the river is 15 feet deep there.

“The reason we have this datum at all these sites is it’s hard to put out big numbers,” Johnson said. “So (418.79) is harder for us to show or harder for people to grasp — or maybe it isn’t, I don’t know honestly — than a number like 10.”

At New Athens, the datum — or arbitrary lowest point — was established at exactly 300 feet above sea level. Today, the water surface is about 370 feet above sea level — or a stage of 70 and a flood stage of 79.6. Why so far out of whack? Someone may have been a little lazy, Johnson joked.

“Someone a long time ago probably said, ‘I don’t feel like trying to figure out how low this river might get but if we know that the elevation of the water surface is now 370, let’s just say the datum is 300. That’s an easy round number people can remember.’”

So, ever since, the river stage has been measured against this arbitrary 300 datum. Such a system can lead to comical results, Johnson said. Chicago, for example, uses a single city datum to measure everything in the area, which means you often find the Chicago River with a negative depth.

“Ideally, we would have known the real-world elevation of all these rivers and we would have just used it,” Johnson said. “So we would put out today that New Athens is at 370.11 with a flood stage of 379.6 The problem is we didn’t do that from time immemorial so we’re left with these aftereffects.”

Today’s trivia

What award-winning actor was the original voice of Charlie the Tuna on Starkist commercials and the Jolly Green Giant?

Answer to Wednesday’s trivia: Here’s a piece of historical irony you may not have realized: Key West, Fla., the southernmost city in the continental United States, is usually said to be the only Southern city to remain in Union hands throughout the entire Civil War. Even though Florida seceded and most locals were rebel sympathizers, the presence of Fort Zachary Taylor, which had been built during the 1850s, prevented the area from falling into Confederate hands. At the very outset of the war, Union Capt. John Milton Brannan seized control of the fort and used it as outpost to threaten those trying to sail through the Northern blockade. To strengthen the fortress, the East and West Martello Towers were built and connected to the main fort by railroad to move munitions. The final straw was the arrival of Maj. William French and his troops from Texas on April 6, 1861.

“Some who had pretended to be in sympathy with the South, saw on Major French’s arrival the destruction of all hope of Key West being a part of the Confederacy, and they became very loud and offensive in their so-called loyalty to the Union,” Jefferson Browne wrote in his 1912 book, “Key West: The Old and the New.” “They spied upon the homes of Southern sympathizers and reported to the military authorities every action that their eyes could ferret out, and sought to have them locked up in the fort.”

This story was originally published September 16, 2015 at 8:10 AM with the headline "Understanding river stages can leave your head swimming."