About 1 o’clock Friday morning, the Mississippi River will reach the highest level ever recorded at Clarksville, Mo. It will keep rising.
For the next two days, the river will go higher than it ever has been. Businesses from First Street to Third Street will be inundated. Levees on an island opposite Clarksville will be overtopped. Farmers and merchants and high school athletes will fill and stack sandbags. The river will continue to swell.
Clarksville’s record flood level was set in 1993. That was what bureaucrats and developers confidently call a “500-year flood.” Now, 15 years later, the water is even higher; it’s heading downstream at a brisk walking pace, flooding one town after another.
If there can be any good news about a flood, it is that, unlike 1993, this time the Missouri River isn’t raging, too. Once high water reaches the confluence just above St. Louis, the generally wider Mississippi River channel should absorb it.

The bad news is that high water will test aging levees in the Metro East. River levels should peak just a whisker below 40 feet in St. Louis early Saturday morning. Forty feet is a kind of magic number.
At 40 feet, water is under enough pressure to boil under or tear straight through levees from Alton to Columbia. Those aging structures, some built as far back as the 1920s, no longer are certain to protect vast swaths of river bottom in the Metro East, home to tens of thousands of people and billions of dollars in buildings and property.
Levees sometimes fail because water levels rise over them. But more often, as New Orleans learned during Hurricane Katrina, water burrows a tunnel under or through them. That water can overwhelm aging pump stations and undercut the levee’s structural integrity, causing it to collapse.
If all goes as forecast — if no rain falls here or upstream between now and Saturday and flood protection on the Mississippi River continues to hold, and if 40 feet is, in fact, the magic number, and not 39 feet — the Metro East will remain relatively dry.
If not?

During the catastrophic flood of 1903 that inundated most of the Metro East, the Mississippi River reached only about 38 feet at St. Louis. That was an exceedingly rare event in those days.
From the time Pierre Laclede helped found St. Louis in 1764 until World War II began in 1941, the river reached 39 feet in St. Louis on only two occasions. Since then, it’s reached that height eight times — nine if the forecast for Saturday holds.
River height is one figure watched closely by scientists and engineers. It’s the figure you hear cited most often, memorialized with high-water marks and used to help determine so-called 100-year and 500-year flood levels.
But another important measure is river flow. That’s the volume of water passing by a certain point, expressed in cubic feet per second. Flow is more difficult to measure than height. But it turns out to be more important when you’re comparing river levels over time.
Back in 1837, when Robert E. Lee mapped the St. Louis harbor, the Mississippi River was 4,000 feet wide at St. Louis. At that time, the river had exceeded 39 feet only once, in 1785 when it hit 42 feet.
Today, the Mississippi River is slightly more than 1,500 feet wide at St. Louis. Levees have been built along its banks to contain it. Navigational structures such as wing dams and chevrons have been constructed along its channel to concentrate its force.
During that 1903 flood, about 1 million cubic feet of water per second was moving past St. Louis. But the river’s height was slightly lower than it’s expected to be on Saturday, when the flow will be only about 760,000 cubic feet per second.
So what happens next time the Mississippi River bulges with a million cubic feet of water per second? Disaster.

That’s the lesson of 1993. And of 1995. It’s the lesson of 1973, 1982, 1983 and 1986. Flood-plain development is a bad idea; higher and higher levees upriver only bring higher and higher water levels downriver and upriver, too.
But it is a lesson unlearned. Since the 1993 flood, development has consumed huge amounts of Missouri River flood plain in west St. Louis and St. Charles counties. New malls, industrial parks and even homes are protected by levees that we arrogantly assert are strong enough to withstand a 100- or 500-year flood. Perhaps they are. But cutting off flood plain gives the river less room to expand. That can cause potentially disastrous changes on both the Missouri and Mississippi rivers that are difficult to predict.
The truth is that we don’t know how damaging a 500-year flood might be. Even if historical records stretched back that far, the river system has been so thoroughly altered by levees and wing dams that the old records largely are irrelevant.
River towns like St. Louis and Chesterfield, St. Peters and tiny Clarksville exist at the confluence of gigantic forces beyond their control.
On one side lies everything we know about engineering and history. On the other is all we don’t know about how we’ve altered the river’s topography and natural history.
In between are a tired crew of farmers and merchants and high school athletes, filling and stacking sandbags — hoping that when the flood comes this time, they won’t all be washed away.

Clarksville residents battle rising water in 1993. Post-Dispatch file photo.