OAKVILLE, Iowa — The floodwaters that deluged much of Iowa have done more than knock out drinking water and destroy homes. They have also spread a noxious brew of sewage, farm chemicals, and fuel that could sicken anyone who wades in.

Yesterday, Bob Lanz used a 22-foot aluminum flatboat to navigate through downtown Oakville, where water reeked of pig feces and diesel fuel.

“You can hardly stand it,” Mr. Lanz said as he surveyed what remained of his family’s hog farm. “It’s strong.”

The chairman of emergency management and homeland security in nearby Des Moines County, LeRoy Lippert, warned people to avoid the floodwaters: “If you drink this water and live, tell me about it. You have no idea. It is very, very wise to stay out of it. It’s as dangerous as anything.”

In addition to the poison in the water, there are mosquitoes — millions of them spawning in acres of standing water.

As some of Iowa’s flooded towns began cleaning yesterday, others braced for new flooding risks, particularly in southeastern Iowa along the Mississippi River. Most requests for state aid were coming from Des Moines County, where the Mississippi was not expected to crest until tomorrow. The county had asked for a half-million sandbags.

“We have just begun to fight,” Governor Chet Culver said. Two more deaths were reported yesterday, including a woman whose car was hit by a National Guard truck, bringing the state’s death toll to five.

Elsewhere, damage in the college town of Iowa City appeared limited. Some 400 homes took on water Sunday, and 16 University of Iowa buildings sustained some flood damage over the weekend. But the town’s levees were holding and the Iowa River was falling.

Officials in Illinois were building up the approach to the only major bridge over the Mississippi River linking Hamilton with Keokuk, Iowa, so the bridge could stay open despite rising water.

In Cedar Rapids, hazardous conditions forced officials yesterday to stop taking residents into homes where the water had receded. Broken gas lines, sink holes and structural problems with homes made conditions unsafe, a city spokesman, Dave Koch, said.

Frustrations spilled over at one checkpoint, where a man was arrested at gunpoint after he tried to drive past police in his pickup truck.

Warnings about the dangers of walking in the polluted water prompted hundreds of people to line up at a downtown clinic Sunday for free tetanus shots.