GAUGING THE MISSISSIPPI; READING IS NOT THE ACTUAL CHANNEL DEPTH

On Saturday morning the National Weather Service reported the Mississippi River gauge at 13.6 at Cape Girardeau.

But the actual depth of the river at midstream, well out from the floodwall, was probably 30 feet from the surface to the river bed.

Why the discrepancy?

"There is no discrepancy," said Bill Bryant, a hydrologist with the weather service at St. Charles, Mo. "The actual depth of the water in the river and the river stage are completely different."

Which also explains how barges can be observed floating up or down the river when there are minus readings.

"Even with the minus reading, the river may have eight or 10 feet of water in the middle," said Bryant. "It depends on where the gauge is located."

The gauge tells you how deep the river is at that point. "But it may be deeper or more shallow a short distance upstream or downstream from where the gauge measured the water," said Bryant.

An article in a December issue of a 1963 Southeast Missourian, described it best.

"For scientific accuracy in measuring over a long period of time, the river gauge is set in a fixed height above sea level and not on the bottom of the main river channel."

The Mississippi River course changes over a period of years, while the deepest part of the river bottom also varies, making it difficult for shore observers to record the accurate river depth without a fixed base.

When a new river gauge was installed on the Mississippi River at Cape Girardeau in 1904, it was described as a "sloping variety" gauge, running from the bank slanting into the river, far from the bottom, with marked divisions of feet.

Over the years, the U.S. Army Corps of Engineers has used dredges to maintain a 9-foot channel in the river form Cairo, Ill. to St. Louis, with only minor stoppages because of river depth.

One of those stoppages occurred in December of 1989 and January of 1990, when the river stage here dropped to a low of 1.29 feet. At the same time, the stage was a minus 5.1 at St. Louis. The river still contained water, and traffic was still moving until extreme cold and ice conditions forced closing of the river for a brief period.

That 1.29 reading on Dec. 28, 1989, was not the lowest at Cape Girardeau. The record low dates back to a minus .16 reading on Jan. 15, 1909, during icy river conditions.

Other low readings, all during heavy ice periods, were 2.6 on both Jan. 7, 1911, and Jan. 2, 1918, and 2.7 feet on Dec. 23, 1922.

Bryant explained that when river gauges came into use at locations up and down the Mississippi in the early 19th century, each one was unique and stood alone.

"Each gauge applied to the local vicinity," he said. A zero point on any given gauge was determined by research on the local scene to determine the lowest known level the river had ever reached at that location.

In any event, those zero points indicated the lowest river stages known at that time. They were never intended to indicate zero water, or the bottom of the river.

Initially the only people who were seriously interested in how much depth existed below the zero point on a local gauge were river pilots.

Later on, surveys were made at each gauge location to determine the absolute elevation of the gauge's zero point above sea level. The zero point of the Cape Girardeau gauge, which is now situated near the floodwall east of the Merriwether Street pump station, is 304.77 feet above sea level.