Floodwaters that breached a Missouri River levee near the Missouri-Iowa border are approaching a makeshift floodwall hurriedly built to protect the small Iowa town of Hamburg from the river's creeping advance.

The water, which breached the primary levee on Monday, was expected to reach the temporary structure guarding Hamburg, five miles to the northeast, sometime Thursday, Robert Michaels, the Army Corps of Engineers official who has overseen construction of the new levee. The new levee was finished Wednesday, and most of those living in the threatened parts of town have cleared out.

Meanwhile, any hopes the breaches might alleviate the long-term flooding threat for communities downstream have been short-lived, as river levels that dipped slightly from the release of pressure began their re-ascent Wednesday.

The river has been rising for weeks as the corps releases increasing amounts of water from its upstream dams to clear out heavy spring rain and snowmelt. Releases at Gavins Point Dam in South Dakota hit the maximum planned amount of 150,000 cubic feet of water per second Tuesday, and the corps wasn't planning to reduce the amount it's releasing from its dams until August at the earliest.

Parts of Montana, North Dakota and South Dakota already have been flooded, and towns and cities farther south were still bracing for the worst.

South Dakota officials estimate the state already has spent about $10 million (euro7 million)in the first two weeks of work to prevent flooding in Pierre, Fort Pierre and Dakota Dunes.

River towns and cities in Nebraska, Iowa and Missouri continued bolstering their own flood defenses Wednesday, probing for any potential weakness.

Officials predict the river downstream of the six dams will remain 5 to 7 feet above flood stage at most places in Nebraska and Iowa, and swell as much as 10 feet above flood stage in Missouri.

John LaRandeau, a civil engineer with the corps, said the situation along the Missouri River is distinctly different than the one along the Mississippi River earlier this spring, when the corps intentionally breached a levee south of Cairo, Ill., to reduce the risk of flooding in that city. In that case, the Mississippi water levels were expected to recede soon after the breach, whereas the Missouri levels are not.

"The pressure on the levees is going to be very, very high," he said.