Crest Measurement Is Not What It Seems: 48 Is 60 Feet

As residents along the Mississippi River frantically prepare for next week’s record crest of 48 feet here, there is considerable confusion about what the measurement actually means.

A farmer fighting the surging river near a broken levee near St. Louis said confidently that it was the depth of water measured from river bottom to the surface. A volunteer sandbagger in south St. Louis said the same.

Not so, says the U.S. Army Corps of Engineers.

River measurements are based on a historical standard established more than a century ago and do not attempt to reflect the true depth of the water. When the river crests at 48 feet, its actual depth will be closer to 60 feet, engineers say.

“It is a very arbitrary number,” said Bill Groth of the Army Corps. “A lot of people get it wrong.”

At the riverfront gauge in downtown St. Louis, considered one of the most important measuring points because it lies near the confluence of three rivers, a zero reading still leaves about 12 feet of water in the river, engineers say.

Indeed, the record low reading in downtown St. Louis was minus 6.2 feet in 1940. The river fell as low as minus 5 feet just a few years ago.

A zero measurement at the downtown arch equates to an elevation of 379.94 feet above sea level. The river’s ebbs and flows, including its flood stages and crests, are then calculated above and below that benchmark.