ALAN SCHER ZAGIER and DAVE KOLPACK

Associated Press

MINOT, N.D. - The reservoirs are full. The dams are open wide. The rivers have already climbed well beyond their banks. Throughout the Missouri River Valley and other parts of the upper Midwest, there's simply no place left for any more water.

That brings a new threat to the nation's water-logged midsection: more rain. In a region already struggling with historically high water, the return of heavy storms could intensify the flooding and turn a soggy summer into a tragic one for a dozen states that drain into the Missouri.

"We know what's coming down the river, and what's going to continue to come down the river," said meteorologist Wes Browning of the National Weather Service office in St. Louis. "But what we don't know with any certitude, beyond five to seven days, is the amount of rainfall. That's really going to drive this flood."

The peril began unfolding during the spring, when storms dumped an unexpectedly large amount of rain across Montana. That precipitation, combined with unusually heavy snowmelt, caused a vast volume of water to build up behind dams in the United States and Canada.

The Army Corps of Engineers and Canadian authorities have been releasing water through those dams for weeks, inundating many low-lying, mostly rural parts of Iowa, Nebraska, Missouri and North and South Dakota. Now the river valley is saturated, and the arrival of any more water could create an even larger disaster.

Many people expect to spend an anxious summer watching the skies and monitoring forecasts.

Terry Higedick, who farms 2,500 acres in central Missouri's Boone and Cole counties, has had plenty of time to prepare. He sold his excess corn and soybeans or put them in a grain elevator. His heavy equipment has been moved to higher ground.

But despite daily updates on dam releases and sophisticated forecasts, he has few reliable ways to estimate how much rain will fall over the next few months - or how high the floodwaters will rise.

"It makes it impossible to plan," he said. "We're kind of stuck in that mode."

In Minot, the danger came from the Souris River, a little-known channel that flows south from Canada without entering the Missouri River basin. On Thursday, crews worked furiously to raise earthen levees in a last-ditch effort to protect at least some neighborhoods, even as officials acknowledged they could not prevent significant damage to North Dakota's fourth-largest city.

The workers on the levee and National Guard troops were the only people to be seen in the endangered areas. As many as 10,000 residents, or about one-fourth of Minot's population, evacuated ahead of the community's worst flooding in four decades.

Thursday's effort also focused on protecting critical infrastructure, including sewer and water service. If those utilities were to be knocked out by floodwaters, more evacuations could be necessary. Parts of the city were already under several feet of water, including a trailer park near the river.

The weather service's Climate Prediction Center issued its three-month outlook for rain on June 16. Above-normal rain was anticipated over a large swath of the Great Plains covering much of the Dakotas, Iowa and Nebraska.

In the worst-case scenario, that rain would gush into the already full river system and produce widespread, near-record flooding from Kansas City to St. Louis.

A case in point: a mere 2 to 3 inches of rain last week in northern Missouri pushed the Mississippi River up 6 feet within days near Hannibal. In Minot, the Souris is expected to top a city record set in 1881 by more than 5 feet.

Flood projections in Missouri are similarly dire. In the state capital of Jefferson City, for instance, the predicted crests of 6 feet to 14 feet above flood stage would wash out roads, breach levees, close railroads, threaten power plant operations and shut down major highways. Experts can't say with certainty if water levels will rise that high, but are warning residents to be prepared.

Right now, those projections are simply "good long-range planning information," Browning said. "From this point on, it depends on what's coming out of the sky."

And the threat looms not just in the amount of rain but also its intensity. A half-inch of rain every day for a week would be far different than a severe thunderstorm that dumped 5 inches of rain in a few hours.

Browning compared the two scenarios to a homeowner watering his lawn.

"I could take two approaches. I could take out a sprinkler overnight with a nice steady, slow stream and nothing would go down the curb," he said. "Or I could do it with a fire hose in five minutes. I would get an inch of water in both cases. But the runoff into the gutter would be far more with the fire hose."

In Minot, the city already endured a major evacuation last month, when the Souris rose briefly to threatening levels. Though residents had been warned that another evacuation was possible, the river's second rise after heavy weekend rains shocked many people, including city officials.

"It's just an unprecedented amount of rainfall this spring in the whole basin," said Mark Davidson, a spokesman for the army corps in St. Paul., Minn. "We're all doing our best, but Mother Nature, she's just tough."

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Associated Press Writer Doug Glass in Minneapolis contributed to this report.

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Zagier reported from Columbia, Mo. He can be reached at http://twitter.com/azagier.