TAYLOR, Mo. -- The Mississippi River has spilled out as wide as 16 miles across the low plains of Illinois and Missouri, a chocolate-colored inland sea so vast that it will leave behind a temporary ribbon of swamps even after it recedes, altering human, wildlife and plant habitats for months.
Lingering floodwaters along the Mississippi's 583-mile upper branch likely will delay a speedy recovery and wreak havoc on everything from the reclamation of sunken river towns to the survival of wild grasses, say scientists and engineers girding for the impact. Unlike earthquakes and hurricanes, which spend their fury in a matter of days, the Mississippi's devastation is a quiet, invisible undermining that intensifies as long as floodwaters remain.
Mosquito and mayfly populations already are showing signs of exploding in the saturated bottom lands. River channels are filling with tons of silt. Thousands of birds are expected to stray off the Mississippi flyway during their fall migration. The risk of bacterial disease and drownings will rise among families trying to return to homes eroded by water, caked with silt and filling with river fish and poisonous snakes.
"What's phenomenal is that even after the river crests, this crisis is not over. It's just starting," said Robert H. Stratton, a federal animal biologist who directs the Mark Twain National Wildlife Refuge, a 275-mile-long preserve now completely underwater.
"We've got above normal rainfall predicted until September, so the water will sit in some places as long as late August and September before it drains out," said Gary Dyhouse, chief hydrologist for the U.S. Army Corps of Engineers in St. Louis. "I wouldn't be surprised to see some water still around by winter."
From the air, the flood's sway over the Mississippi valley is clear. North of the town of Quincy, Ill., the river bulges out to its greatest width, 16 miles of water lapping at the steel tops of 30-foot-high grain silos. Another muddy 15-mile band stretches further south, at Clarksville, Mo. Even beyond those widening banks, small lakes and and curling fingers of ground seepage splay for miles -- evidence that standing water will not soon evaporate.
Beneath the swirling surface of the Mississippi, the strong 15-mile-an-hour current is shaving and planing the river bottom, altering the depths of boating channels. Buoy markers have been swept away "like match sticks," said Army Corps spokeswoman Denise Yale.
"The face of the river will change," Ms. Yale said. "Boaters will have to relearn the river. Debris is changing the shoreline. We'll get new sandbars at the bends."
The corps already expects it will have to dredge some of the river's bottom to maintain the Mississippi's standard 90-foot depths. And in some flooded areas, farmers will be forced to dynamite levees to drain off standing water as the river level recedes.
Standing water already has begun to kill off all but the most durable vegetation by cutting off sunlight essential for photosynthesis.
The damage to washed-out crops is evident, already being calculated in the billions of dollars. But the bare ground that remains also will likely have a stunning impact on this fall's southern migration of teal, mallards, canvasback ducks and Canada geese, according to wildlife experts.
Normally a major flight corridor in the fall, the Mississippi's farms provide sustenance for southbound bird populations. But this year, bare fields left by the flood will drive migrating birds away from the river, say Mr. Stratton and other experts.
Decomposing animal corpses add to the disease risk in the Mississippi, already churning with sewage, farm pesticides and chemicals. Hydrologists say the massive volume of the flood has diluted the ecological risk posed by toxic runoff.
But disease risk lingers. Last week, Larry Barker, the Davenport, Iowa, public health director, went out to the city's downtown flood zone to make a televised appeal to watch out for disease risks. As he spoke, he watched dead fish, rusty nail-studded boards and fecal matter float by.
"It was a graphic illustration of the problems we could have," Mr. Barker said. "I didn't even have to say anything."
Public safety officials say sewage spewed out by flood-jammed river city waste treatment plants from St. Paul, Minn., to St. Louis poses a major health risk. In St. Paul, where flood waters are still receding two weeks after cresting, coliform bacteria counts have topped 1,600 particles per million -- above the normal safe range of 1,000 parts per million, said Assistant Fire Chief Warren Schaub, who directed flood operations.
When Mr. Stratton returned from a flight over the roiling Mississippi yesterday, he had only to stroll around the tiny airstrip to find signs showing this flood's expected long incubation.
Mayflies coated an airstrip trailer, basking in the weak morning sun. On the farm field bordering the grass runway, shallow depressions brimmed with ground saturation. The sky above was blank, gray and birdless.
"A hurricane is over in 12 hours," Mr. Stratton said. "Twelve months from now, we'll still be feeling this."