The great Mississippi River flood of 2011, cresting south of Memphis today, carries a mix of fertilizer, oil, pesticides, trash and farm runoff as it flows toward the Gulf of Mexico, say public health officials.
Some of it is nasty stuff, and officials say people are wise to be careful. They urge people not to touch the water unless they're wearing rubber boots and gloves, and wash thoroughly if they get wet.
"There could be a lot of untreated sewage coming downstream," said Wilma Subra, an environmental scientist and activist in Louisiana who has tangled with oil and chemical companies. "People need to be aware."
ABC News arranged some testing of its own, taking water samples from two places along the river to a laboratory near Memphis. E. coli and coliform -- commonly found in untreated waste water -- were 2,000 times acceptable limits. The lab did not find gasoline, oil or chemical toxins. There were trace levels of heavy metals, but no more than would be found ordinarily, the lab reported.
Subra said she would be concerned if the giant Morganza Spillway were opened upriver from New Orleans and Baton Rouge. It would protect the cities, but flood the wetlands of southern Louisiana. And it could be a health issue as people return to flooded homes to clean up.
"When in doubt, throw it out," said the Tennessee Department of Health in an advisory to people trying to clear out their homes when the water goes down. "Flood water picks up numerous contaminants from roads, farms, factories and storage buildings, including sewage and chemicals."
The state also warned that standing water provides a perfect breeding ground for mosquitoes. "It's very important that mosquito repellents be used and other precautions be taken to protect individual health," said Abelardo Moncayo, who directs the health department's vector-borne diseases program.
Perhaps the largest effect: the overflow of nutrients into the Gulf of Mexico is likely to create an unusually large "dead zone" -- a giant patch of water off the Texas-Louisiana coast where fish and other marine creatures lack enough oxygen to survive. A dead zone forms there almost every July and August, but scientists said it will be bigger this year because algae, feeding on the excess fertilizer, will bloom and then die, choking off the oxygen supply.
"We know that any time we have a lot of rain up here, that's when we have a large dead zone," said Mark David, a professor of environmental science at the University of Illinois.
It is bad news for fishing families, still trying to recover from the effects of last year's BP oil spill, who will have less to catch to make up for their losses.
Cities and towns in 31 states use water that flows into the Mississippi River Basin, many of them releasing treated wastewater into tributaries of the Mississippi. Engineers worried that sewage treatment plants could be overwhelmed by floodwaters.
David said he was not very concerned: "We protect things pretty well along the Mississippi because we know it floods."
Several scientists said to keep the pollution issue in perspective. With large volumes of water rushing downriver, they naturally dilute any pollutants they carry.
"I think it's the economic damage that's the biggest issue," said Nancy Rabalais of the Louisiana Universities Marine Consortium. "As long as people are vigilant, it won't be a people-living-in-the-water issue."
ABC News' Steve Osumsami and Sarah Amos contributed information for this story.