On Wednesday morning the National Weather Service reported the Mississippi River gauge at Cape Girardeau was at 46.7 feet, with the river on its way to a crest of 47.2 feet on Saturday.

But the actual depth of the river at midstream, well out from floodwall, was 65 feet from the surface to the river bed.

Why the discrepancy?

"There is no discrepancy," said Claude Strauser, potomologist with the Corps of Engineers' St. Louis District office. "The actual depth of the water in the river and the river stage are completely different; they're apples and oranges.

"The depth of the water is just that: it 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 you measured the water depth," Strauser explained.

"A better way was needed to measure the amount of water in the river to help the early steamboat pilots navigate the Mississippi before we had the present 9-foot navigation channel. That's why river gauges were developed."

Strauser said 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 and was referenced elsewhere only in the minds of the river pilots," said Strauser. "The 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.

"It has been said, with unknown authenticity, that the most common source for this information was the memory of the oldest local resident," Strauser said. "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. Again, initially at least, the only people who were seriously interested in how much depth existed below the zero point on a local gauge were the river pilots."

Later on, Strauser said that surveys were made at each gauge location to determine the absolute elevation of the gauge's zero point above sea level. He said the zero point of the Cape Girardeau gauge, which is situated near the floodwall east of the Merriwether Street pump station, is 304.65 feet above sea level.

"Basing the zero point on the gauge's elevation above sea level provides a reference base for relating one gauge with another," Strauser said. "And some river gauges now read directly in absolute elevation above sea level and have no zero on them. An example is Mosier Landing, Ill., which has a flood stage of 441 feet above sea level."

As another example, Strauser said that all lake stages such as those at Kentucky, Barkley and Wappapello lakes are measured in height above sea level.

"The advantage of absolute (sea level) gauges is the direct relation to other gauges and other elevation measurements, such as bench marks," he said.

"All levees are constructed according to protection factors based on height above sea level. Of course, the disadvantage of using sea level height to measure the amount of water in the river is that local residents find sea level figures are more difficult to relate to their traditional way of measuring the depth of the river, which is why we still use both methods to indicate the river stage at any given location."