HAMBURG, Iowa -- Dozens of workers raced Tuesday to add several feet to a levee that now stands as the lone barrier between Hamburg and the floodwaters of the Missouri River that threaten to fill the small town like a bathtub.

Crews working for the Army Corps of Engineers hoped to pile at least three feet of extra dirt atop the temporary levee. But time was short and the stakes were high: If the levee were to fail, parts of this southwestern Iowa community could be covered by as much as 10 feet of water within days. And the high water could linger for months.

The construction work stirred up a cloud of dust as teams hurried to complete the improvements by Wednesday evening. The earthen levee became Hamburg's last line of defense after the river punched through another levee downstream in northwest Missouri that provided the town's primary protection.

That failure left water gushing through a large gap on a path to inundate the town of 1,100 -- unless the other levee can be made taller.

"I feel good about it," Fire Chief Dan Sturm said. "But we can't guarantee anything. We've never really had to cope with anything of this magnitude."

Even though the levee breach was downstream, the floodwaters were flowing north to fill the area around Hamburg because the town sits in a valley. The fire chief compared the geography to a slowly filling bathtub.

The water was initially expected to arrive in Hamburg on Tuesday, but the corps later said the flooding would not reach the new levee until sometime Wednesday, giving workers an extra day to finish the job.

Corps leaders expected the effort to add three more feet to the levee to be complete by midday Wednesday.

"You can see the water coming," said Col. Bob Ruch, commander of the corps' Omaha district.

The river has been rising steadily for weeks as the corps increases the amount of water released from its dams to clear out heavy spring rain and snowmelt.

On Tuesday, the releases at Gavins Point Dam in South Dakota hit the maximum planned amount of 150,000 cubic feet of water per second. So officials downstream in Nebraska, Iowa and Missouri were sure to be watching for more levee problems.

The dam releases are expected to raise the Missouri River 5 to 7 feet above flood stage in most of Nebraska and Iowa. In Missouri, the river may climb 10 feet above flood stage in some places and spill over the top of several rural levees.

Parts of Montana, North Dakota and South Dakota have already seen some flooding, and officials predict the problems will linger through the summer because of the large volume of water already in the river and the larger-than-usual Rocky Mountain snowpack.

The corps does not expect to reduce the amount of water released from the dams until at least August.

So far, the floodwaters have covered mostly corn and soybean fields with few structures. But it's an unwelcome development for farmers because grain supplies are at historically low levels and demand is strong for every bushel of corn and soybeans.

Still, the loss of the crops is unlikely to affect overall U.S. production because the areas underwater are relatively small.

Mike Nenneman, a farmer from Sidney, is waiting for the flood to swamp a 360-acre tract of corn and soybeans he owns in far southwest Iowa. He expects to break even, with $700 per acre in crop insurance to offset his losses.

"We are the drain of southwest Iowa," Nenneman said, gesturing to the Missouri River to the west and the Nishnabotna River to the east. "We take all the water from everywhere."

In Hamburg, workers hoped to complete the levee project by Wednesday night. When finished, it will be about eight feet tall in most places.

A line of tractor-trailers carrying dirt to the levee stretched for more than a quarter-mile Tuesday morning. Once the trucks reached the work area, tractors and other earth-moving equipment carried and pushed it to the levee.

To help buy some additional time for the levee work, the corps cut a 300-foot-wide, three-foot-deep notch in the same Missouri River levee south of Hamburg that recently failed.

The notch will allow some of the floodwater to drain back into the river, but will only slow the water's advance toward Hamburg, Ruch said.

The corps started building the new Hamburg levee last week after finding problems in the main levee in Missouri.

Several businesses near the levee stood empty Tuesday as crews toiled on the new levee.

Todd Morgan with A&M Green Power Group says the owners of the John Deere dealership moved their business to one of the company's other dealerships in Shenandoah, 25 miles away.

"We wanted to play it safe than sorry," Morgan said. "Every day that goes by, you seem to hear something different. With the breach yesterday, we just don't know what the integrity of that levee is."

Morgan said he doesn't know whether the dealership will return.

Fremont County Sheriff Kevin Aistrope said all but seven of the roughly 40 households in the southern part of Hamburg have evacuated voluntarily. The remaining seven have moved all of their furniture and can escape quickly if water floods the town, he said.

About 45 miles south of Hamburg in Missouri, the river also broke through a levee near Big Lake in Holt County. About 30 residents who had stayed in the resort town after the river started rising were told to leave Monday, but some refused to go.