Missouri's Black River levee fails. Where floodwaters could hit next.

Residents along the Mississippi and Ohio Rivers brace for a pair of crests that could bring record floods. In Missouri, 1,000 flee the overflowing Black River after 15 inches of rain in four days.

Record or near-record flooding is expected along stretches of the Ohio and Mississippi Rivers in the coming days as the central and south-central US struggles to cope with the peak of the 2011 flood season.

Record winter snows and prolonged cold that kept the ground frozen and unable to absorb the spring melt have combined with a week of pummeling rains from intense spring storms to flood fields and riverside communities.

The spring storms, which have brought tornadoes as well as floods, have prompted the governors of Arkansas and Kentucky to declare states of emergency.

In Poplar Bluff, Mo., which has received some 15 inches of rain over the past four days, levees protecting the town from floods on the Black River have been breached or overtopped in several places, forcing the evacuation of some 1,000 people. According to the Associated Press, the privately maintained levees that protect homes and farms near Poplar Bluff failed an Army Corps of Engineers inspection in 2008.

In the meantime, federal hydrologists are keeping a close eye on two flood crests – one working its way down the Mississippi and one on the Ohio River, which empties into the Mississippi at Cairo, Ill.

The Mississippi's largely snow-melt driven crest is expected to pass Cairo first, followed closely by the crest from the Ohio. The two are expected to move south in tandem along the lower Mississippi.

Over the next two days, this one-two punch is expected to bring record or near-record flood levels to a stretch of river from Smithland, Ky., on the Ohio to New Madrid, Mo., on the Mississippi, according to Noreen Schwein, deputy for hydrological services at the National Weather Service's Central Region Headquarters in Kansas City, Mo.

Officials with the US Army Corps of Engineers were sufficiently concerned about this pending aquatic onslaught that they were considering blasting a hole in a levee along the Mississippi just south of its junction with the Ohio. But they reportedly put off a decision after Missouri's attorney general went to court to block the move, arguing that some 130,000 acres of farmland would become inundated.

Hydrological 'perfect storm'

The challenge presented by a pair of closely-spaced, slow-moving crests is that they tend to back water up behind them, prolonging the time that levees are under stress and that communities upstream continue to face flood conditions, says the National Weather Service's Schwein.

To be sure, the Midwest and Upper Plains states are no strangers to severe spring floods. This year's flooding, however, is the culmination of a hydrological "perfect storm," meteorologists say.

Heavy snow with high moisture content blanketed much of the northern US this past winter, with temperatures cold enough to keep the ground frozen well into melt season. The snow began melting before the ground could thaw and absorb the meltwater. Instead the water flowed straight into the streams and larger tributaries that feed rivers such as the upper Mississippi and the Red River of the North.

Ms. Schwein notes that the situation along these rivers could have been worse. In the north, some of the spring precipitation has come as snow, rather than rain. Because the snow takes time to melt, the additional water is introduced into the region more gradually than if the precipitation had come as rain.

As heavy spring floodwaters moved through upper Mississippi basin and into the storied river itself, a series of powerful storm systems moved through the country to the south, dumping heavy rains on soils that quickly became saturated. The storms also have triggered flash floods in the Ozarks, as well as in parts of western Kentucky and southern Illinois.

As of Tuesday afternoon, tornado watches had been posted for a stretch of the country running from northeastern Texas through western Tennessee, and from the northern half of Indiana through much of lower Michigan. Flood and flash-flood watches covered much of the same area.

Flooding is setback for farmers

April's onslaught is taking a toll not only on homes and businesses along the nation's inland waterways. The floods are striking at the country's farmers as well.

Jeff Scates, who along with 11 other family members farms 15,000 acres near Shawneetown in southern Illinois, says that before the rains came, he had 30 percent of his corn crop planted.

But the heavy rains changed that.

"In a few days, we'll be about 15 percent planted," he says, noting that the rest of his pre-rain efforts have fallen casualty to the storms and flooding. "Normally, we'd like to be about 50 percent planted by now."

Within the next two days, the Ohio River at Old Shawneetown, not far away, is expected to reach 55 feet above flood stage, the fourth highest level on record there.

"That's about a half inch below the second-highest level on record," he adds.

Nationally, corn-planting is far behind schedule, according to the US Department of Agriculture. Typically, over the past five years, farmers have seeded an average 23 percent of their corn crop by now. Last year that number reached 46 percent. This year, the wet weather has allowed farmers to plant only about 9 percent of their planned corn acreage. Some agriculture economists say they suspect that even the 9 percent figure could be too high.

Corn represents about 60 percent of his family's plantings, Mr. Scates says. With May around the corner, it's unclear whether the ground will dry fast enough – and stay dry long enough – to get seeds into the ground. After about May 20, he says, each additional day's delay in planting means another 1 percent loss in crop yield.