The snakes are out. And the bears. The gators. The jumbo rodents known as nutria. The feral hogs. They seek higher ground as the floodwaters advance, and that can mean the top of a levee or in someone’s back yard. Herds of deer have clumped on tiny islands in an ancient swamp that is becoming a lake.

The humans are scrambling, too. They’ve filled a million sandbags. The flood fighters deploy barriers known as tiger dams, HESCO baskets, aqua tubes and sheet pile. The hospitals have stockpiled antivenin in anticipation of a surge in snakebites. Officials the other day shot a 10-foot gator on a levee near New Orleans.

Engineers on Saturday were tracking roughly 250 seepage points along the levees that line the Mississippi and Atchafalaya rivers. The levees sit on fine sand that lets the river water escape like a convict tunneling out of prison. The water can pop up a mile away. Left to its own devices, one of these sand boils (where water erupts as if from spring) can undermine a levee and lead to a crevasse, a full levee failure — and disaster.

There’s a war underway, fought across the Mississippi’s 35,000-square-mile alluvial plain. This is the toughest test since 1973 of the Army Corps of Engineers’ flood-control system along the Mississippi, which drains 41 percent of the land in the lower 48, and is now swollen by snowmelt from the Rockies and rainwater from the soggy Ohio River Valley.

For the moment, engineers seem to have the upper hand. They are not claiming victory, however. This is not simply a big flood but also a long flood, one that will last well into June. A flood exists in four dimensions, including time — because a long-duration flood can be more problematic than one that crests and recedes quickly.

“Everything that is part of our toolbox is in use,” reports Col. Ed Fleming, New Orleans district commander for the Corps. “There’s no doubt there is going to be a long crest.”

His colleague Mike Stack, chief of emergency management for the Corps at New Orleans, says, “The system is under tremendous stress, and it’s going to be that way for a while.”

Stack added: “It’s performing as it’s designed.”

‘Dealing with earth’

But as with any complex system of engineering, there are weak points, question marks, vulnerabilities. Powerful forces are being checked with levees made of clay.

Four barges carrying Midwest grain broke loose Friday in Baton Rouge, and two of them sank. That shut down the river for five miles and kept officials fretting well into Saturday as they worried that one of the barges might plow into a levee and create a breach.

“That system is designed to handle the river and the pressure of the river. It is absolutely not designed to handle a barge hitting it,” Steve Wilson, president of the Pontchartrain Levee District, said Saturday.

“We’re not dealing with digital technology. We’re dealing with earth,” said Joseph Suhayda, a retired Louisiana State University coastal hydrologist. “This goes back to the beginning of civilization. It’s available, it’s cheap, but it’s not very good material.”

He went on: “These seepages and sand boils are reflections of the fact that there are some continued deficiencies in the system. This is not a robust system. It’s not concrete.”

The Army Corps has long prepared for a hypothetical inundation known as the Design Flood. This flood pretty much fits that template. The flow, measured in cubic feet per second, isn’t quite at Design Flood levels, but there are places where the river gauges have measured record-high water, busting the old mark by three feet in some spots.

For years, the smart money has bet that, in the protracted wrestling match on the Mississippi between man and nature, nature will ultimately come out on top. The decision, going back to the 19th century, to imprison a naturally meandering river between levees — parallel Great Walls of China, to use the common analogy — has the inevitable effect of raising the water level downstream. Even at normal stages, the river stands up “like a vein on the back of a hand,” as John McPhee wrote in his 1989 best-selling book “The Control of Nature.”

Staying vigilant

Levee failures can be killers. National Guards now walk the levees day and night. They want to pounce on small problems before they become big problems.

A week ago, an enormous sand boil appeared just north of the Louisiana border, in Arkansas. It measured more than 120 feet across. Soon surrounded by sandbags, it became an instant swimming pool. The downward pressure of the pooled water counterbalanced the pressure from the nearby river.

“I’m feeling vigilant. Literally saying prayers every day,” said Garret Graves, chairman of the Louisiana Coastal Protection and Restoration Authority. “The potential disaster here is real, and we’re doing everything we can to make this go down in history as simply a record-high-water event rather than a record disaster.”

The system is designed to limit the river stages at Baton Rouge and New Orleans during a major flood. The solution is diversion. First, the Corps opened the Bonnet Carre spillway north of New Orleans, dumping a portion of the river flow into Lake Pontchartrain. Then the Corps opened the Morganza spillway, north of Baton Rouge. That relief valve, completed in 1954, has been opened only once before, during the 1973 flood.

The moves do not come without a cost. Fresh water is surging into normally brackish Lake Pontchartrain, with potentially devastating effects on the marine life there. And to the west, the Atchafalaya River basin is slowly filling with water diverted at Morganza. Residents have evacuated, some unhappy to have their lives disrupted to spare the big cities and industries to the east.

Day by day the water surges Niagara-like through the Morganza spillway and advances through the kingdom of crawfish.

Perhaps the trickiest part is ahead: Once the water hits Morgan City, it will be pinched by levees and floodwalls and will begin to “stack up.” As the water rises in the floodplain, it will find ways to spill into adjacent areas in a process known as backflooding.

For example, Bayou Chene is a major concern for public officials and residents in low-lying communities. Floodwater will sluice past the high walls of Morgan City on its way to the gulf, and then take a sharp left, into Bayou Chene, and attempt to flow back north, into a lake that could swamp Morgan City from the rear.

To stop that from happening, engineers have sunk a barge, 480 feet long, in the mouth of Bayou Chene. They’ve rammed sheets of steel in front and back, and flanked it with giant boulders. The top of the barge juts above the water line. It’s an emergency dam, a fat finger in the dike.

“This is not something that somebody drew up on the back of an envelope. Engineers thought this through,” Fleming said.

Environmental impact

The chemistry of that floodwater is also raising concerns. Anna Hrybyk, program manager for an environment organization called the Louisiana Bucket Brigade, said the floodwaters could inundate hundreds of abandoned petrochemical waste pits.

“Some of these waste pits are like a New York City block long. They’re huge. And they’re not covered. And they’re not lined. Imagine 25 feet of floodwaters running over these things.”

The ultimate fear for the Army Corps is that the Mississippi’s primary distributary, the Atchafalaya, could capture the majority of the river flow, which could devastate the shipping and petrochemical industries on the lower Mississippi. Preventing the Atchafalaya from doing that has been the job of an elaborate apparatus called the Old River Control Structure, which is not far upriver from the Morganza spillway. During the 1973 flood, that structure was badly undermined by churning water. It was to avoid a repeat of that incident that the Corps opened the spillway gradually.

Even if the system passes this test, there’s a bigger flood out there, somewhere in the future. Every new parking lot, channelized stream or other drainage improvement in the watershed upriver translates to faster runoff and higher water down in the delta. And climate scientists will point out that a hotter planet can carry more moisture in the atmosphere, leading to intensified deluges.

Graves said the system may be working now, but it’s still not ideal.

“It’s a good system. Is it a great system? No. We need more options, ultimately. We need more relief valves. This thing is literally being tested to its rim. It’s not a comfortable feeling,” he said.

After a beat, he added: “Is that the understatement of the year?”