FINALLY, CREST WILL HIT TODAY; RIVER SHOULD REACH 46-6 FEET ON GAUGE

When the Mississippi River crests today at an expected 46.6 feet, it will set a new all-time record flood stage at Cape Girardeau.

It won't be a record flood in terms of water flow or velocity, according to the U.S. Army Corps of Engineers. That record - set 149 years ago - will still be intact after the 1993 floodwaters go down.

The previous flood crest prediction of 47.2 feet on Saturday was revised downward on Thursday by the National Weather Service.

The river at Cape on Thursday was 46.5 feet, a drop of .2 of a foot. But weather service hydrologist Jack Burns said the fall was only a minor20fluctuation, and the crest is still expected at 46.6 feet today. It is expected to drop back to 46.5 on Saturday and 46.4 feet on Sunday.

The Ohio River at Cairo, Ill., was at 44.9 feet on Thursday. Flood stage is 40 feet. It is forecast to crest on Saturday at 45.2 feet

Weather service forecasters have warned the Mississippi may not fall below flood stage at Cape until sometime in mid-August. The 30-day river forecast calls for the Mississippi at Cape to drop to 45.8 feet by July 28, 41.0 feet on Aug. 4, 34.5 feet by Aug. 11, and 30.2 feet on Aug. 18. Flood stage at Cape is 32 feet.

Burns said the 30-day forecast levels are the minimum levels expected by the weather service. "Any additional and prolonged significant precipitation after July 21 would raise those levels," he noted.

On Thursday, more heavy thunderstorms and heavy rains were occurring over northwest and western sections of the state, with thunderstorms forecast for the rest of the state through the end of the week. But unless the rainfall is significant and prolonged, Burns said the only impact in the Cape area will be to prolong the flood crest and slow the fall of the river.

No matter what the final crest is this week, the flood of 1993 will go into the record book as the greatest flood on record here. The previous all-time record high flood stage at Cape was 45.6 feet, on May 1, 1973. That record was broken earlier this week when the river here reached 45.62 feet on May 18, at 2 a.m. CDT.

But in terms of water volume, this isn't the all-time flood.

The Corps of Engineers has two ways to measure the water in floods. The first method, by height or flood stage, gets all the attention. The second method is by determining the volume of water in the river moving past a stationary point, such as a city like Cape Girardeau. Corps hydrologists measure this flow in cubic feet per second, or cfs.

If the greatest flood on record at Cape Girardeau were determined by water flow, the flood of July 4, 1844, is the great-grandaddy of them all.

On that day, when the river crested at 42.5 feet, the Corps estimates 1,375,000 cubic feet of water was passing by Cape Girardeau every second.

The second greatest flood in terms of water flow at Cape was on April 20, 1927, with a flow of 1,054,000 cfs, and a flood crest of 40.0 feet.

On Wednesday, the Corps said about 1 million cubic feet of water were passing Cape Girardeau every second, making this flood the third greatest in terms of water flow. The flood stage at Cape at the time was slightly over 46 feet, and rising.

In contrast, during the flood of May 1, 1973, which set the previous all-time record flood crest of 45.6 feet, the water flow at the flood crest was only 886,000 cfs.

That raises the question: Why are flood crests on the river the past 40-50 years higher than they were in 1844? The answer is that in 1844 there were few if any levees along the river. That allowed the river to spread out over its wide flood plain during high water.

During the past 149 years, much of the Mississippi's flood plain has been developed by man, and levees and floodwalls have been built to protect people and property. As more levees were built, the constriction of the river caused flood crests on the river to go higher.

But water flows have remained constant over the same time period, since they are not affected as much by the levees.

Despite breaks that occurred in some of those levees this month, the Corps of Engineers asserts that flooding in the Cape Girardeau area could have been much worse.

Dave Busse of the water control section of the Corps' St. Louis District Office said flood control dams and water storage reservoirs built at Lake Shelbyville and Lake Carlisle on the Kaskaskia River; the Clarence Cannon Dam and Lake, located north of St. Louis; and dams and reservoirs located on the Upper Missouri River Basin north of Kansas City have reduced the magnitude of the flooding in this area.

Busse said the dams and reservoirs reduced the floodwater flow past Cape Girardeau by about 300,000 cfs, or 25 percent. "Had all of this water not been held back, the water flow past Cape Girardeau would have approached, or even surpassed, the 1844 flood level," Busse said.

"If that had occurred, all of the federal agricultural levees south of St. Louis that are designed to protect to a crest of 50 feet would have been overtopped by now."

It cost about $1 billion in 1993 dollars to construct all the levees and floodwalls along the river in the Corps' St. Louis District, Busse said. The includes the floodwall in downtown Cape Girardeau and St. Louis.

"This year alone, the levees and floodwalls have provided $5 billion worth of protection on an investment of $1 billion in today's dollars. That's a pretty hefty benefit-cost ratio," he said.

Although several private levees along the river in the St. Louis District have failed the past two weeks, only one federal agricultural levee - on Kaskaskia Island near Chester, Ill. - had been breached as of Thursday. Bill Busch, who is in charge of the Corps' Cape Girardeau area office, points out that the Kaskaskia levee was not overtopped but failed as a result of a sandboil that weakened the levee.

"The levee was high enough, but there was just too much water on it for too long a period," he said. "The agricultural levees are built to withstand a 50-foot flood crest, but they cannot be designed to have water on them for weeks or months at a time. This year, we've seen water on the levees for a lot longer than normal."

Busch said some of the levees in the Cape area have had water at their base (toe), or even higher for more than 90 days.