They are looking for any droplets that seep through the barriers and any cracks that threaten to turn small leaks into big problems. The work is hot and sometimes tedious, but without it, the flooding that has caused weeks of misery from Illinois to the Mississippi Delta could get much worse.
"I volunteered for this," said jail inmate Wayne McClinton, who was helping with the sandbagging effort in northern Louisiana's Tensas Parish. "It's a chance to get out in the air, to do something different. It's not boring like prison is."
To take pressure off levees near Baton Rouge and New Orleans, engineers have opened two major spillways. After water was released over the weekend at the Morganza spillway near Baton Rouge, deputies and National Guardsmen fanned out to warn residents in its path, most of whom have heeded the call to seek higher ground.
Bernadine Turner, who lives in a mandatory evacuation zone near Krotz Springs, spent a third day Monday moving her things out. Water was not expected to reach the town about 40 miles west of Baton Rouge for several days, but most residents were taking no chances.
"There's no doubt it's going to come up," she said. "We don't have flood insurance, and most people here don't. Man, it would be hard to start all over."
Snow melt and rain have sent a relentless torrent of water down the Mississippi this spring. On Monday, President Barack Obama flew to Memphis, Tenn., to comfort families affected when the river rose last week to within inches of the record set in 1937. Some low-lying neighborhoods were inundated, but levees protected much of the rest.
Downriver in Mississippi and Louisiana, the crews keeping watch on floodwalls and levees included those from the U.S. Army Corps of Engineers, various local levee districts, county sheriffs, municipal police forces and private security details.
"For the most part, it's hot and boring until you find something," Col. Jeffrey Eckstein, commander of the Vicksburg, Miss., district of the Army Corps, said as he toured the river last week.
Although the job requires 24-hour vigilance, Reynold Minsky, president of a north Louisiana levee district, said there are some places in his mostly rural district of forest and farmland where he will not ask anyone to go after sundown.
"Unless we've got a serious situation that we know we've found before dark, we don't ask these people to go into these wooded areas because of the snakes and the alligators," Minsky said while taking a break from helicopter tours of the levees. "That's inhumane."
Minsky's 5th Louisiana Levee District is plagued these days by "sand boils," places where river water has found a way through earthen levees and bubbled up on the dry side like an artesian well. He said they are no reason for alarm. If the water is clear, as it has been so far, that means the levee is not eroding. Stopping the boil involves ringing it with sandbags.
"We've got more sand boils than we've had in recent days, and we're going to have more. We know that," Minsky said. "They're popping up in different places that we've not had them before."
In New Orleans, the wharves that form much of the city's boundary with the river provide one line of defense. If for any reason they are topped by floodwaters, concrete floodwalls and huge metal gates can also be closed.
On Friday, officials with New Orleans' local levee district gathered around a trickle of water seeping from a gravel railroad bed between the French Quarter and the banks of the Mississippi River. The seepage and the men watching it were barely noticed by the throngs of tourists.
Gerry Gillen, executive director of the levee district, said it was no cause for alarm. The water was clear, giving no sign of erosion or damage. It might even have been residual water from a recent rain.
But such signs can't be ignored, and workers inspect the levees daily when the water is high to look for potential trouble spots. The rail bed was in the same area where high water triggered similar worries in 2008.
The river levees at New Orleans -- which are not among those that failed along canals after Hurricane Katrina -- have survived high water before and will survive this latest test, city officials said.
Saturday's opening of the Morganza spillway north of Baton Rouge -- a relief valve that sent water gushing into the mostly rural Atchafalaya River basin -- stopped the river's rise at New Orleans.
It was the first time the spillway had been opened since 1973. The river crested at 17 feet above sea level late Saturday, and it is expected to remain high for weeks, so the constant patrols will remain a necessity.
Those keeping the constant vigil scoff at any talk of stress.
"We're in the emergency management business. It's not a stressful job," said Kelly Greenwood, chief engineer and CEO of the Yazoo-Mississippi Delta Levee Board said. He was in the northwestern Mississippi community of Rena Lara, where trucks sped up and down a dirt road carrying material to reinforce a levee at DeSoto Lake, which is fed by the Mississippi.
Over the weekend, the Port of New Orleans said Coast Guard authorities had concluded that the flooding would not hinder shipping on the lower Mississippi.
The floodwaters could reach depths of 20 feet in coming weeks, though levels were nowhere close to that yet in the towns that lie about 50 miles downstream of the Morganza.