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BISMARCK, N.D. (AP) ― When a Missouri River ice jam just south of this city led to flooding, a private demolition crew and National Guard soldiers planted plastic explosives in the ice and blew away the problem. It isn't always that easy.

Scientists have gathered a wealth of information on river ice jams such as those that threatened communities along the Missouri and Red Rivers this week. But predicting and managing them still is not an exact science -- and experts say the next one could occur at any time.

Roger Kay, an Army Corps of Engineers expert who has two decades of experience fighting ice jams, said the one south of Bismarck that backed up Missouri River water into neighborhoods this week was moderate in size but "very severe" in terms of impact. The flooding forced an estimated 1,700 people from their homes. They have since returned after river levels dropped following the demolition work.

Ice jams are not the main cause of the Red River flooding in Fargo, where residents are battling a record river crest. But they are a problem on the river north of Winnipeg, where they have caused overland flooding.

There are two main types of ice jams. "Freezeup" jams occur during early to midwinter and typically are made of "frazil" or slush ice. "Breakup" jams form in late winter or early spring as a river and its tributaries thaw out, and have ice blocks that can be as big as cars.

The breakup jam near Bismarck was unusual. Since the Garrison Dam was completed more than half a century ago north of Bismarck, "breakup" ice jams on the Missouri River have been infrequent. None were documented in March or April in the Bismarck area since 1960 -- until this year.

Before that, going back to the 1880s, there were 13 documented jams in March or April.

"When you add a dam, the ice stops in the upstream pool," said Kate White, a civil engineer at the Army Cold Regions Research and Engineering Laboratory in Hanover, N.H., and one of the nation's leading experts on ice jams. "What you didn't get anymore was the ice supply that came upstream from the dam."

Authorities said the ice jam that plugged the Missouri south of Bismarck came from ice pouring into the river from the Heart River tributary, which is south of the dam. White said a near-record snowpack in the region, thick ice, sudden warming and then cooling again was "kind of a recipe for disaster."

Ice jams themselves are not uncommon. White said there have been 167 of them across the country so far this year. An ice jam database developed by scientists from several agencies has 16,400 entries dating back more than a century.

White said officials have tried several methods of predicting ice jams but what scientists really need for the job is a full understanding of various site-specific conditions, because ice jams have unique circumstances and characteristics. Time and money are barriers to that, she said.

Faye Hicks, an ice jam expert at the University of Alberta, said gathering information is difficult for researchers because ice jams are dangerous, and they can occur within minutes.

"We don't necessarily know where to be at the right time," she said. "It becomes a big challenge. It's particularly difficult for areas were ice jams are intermittent."

White said climate change caused by global warming likely is changing ice conditions and adding to the unpredictability.

When ice jams lead to flooding, officials have several options for trying to remove the barrier. Explosives -- which were effectively used to carve a channel through the Bismarck ice jam -- are one. Others include salt to melt the ice, also used in Bismarck, or heavy backhoe-type equipment to break it up.

Authorities are using heavy icebreaking equipment north of Winnipeg in southern Manitoba, where ice jams and the rising Red River have threatened hundreds of homes.

But the equipment can do only so much, said Paul Guyader, an emergency coordinator for the community of St. Andrews. This year could be more of a problem, Guyader said, because upstream on the Red, Fargo is bracing for record flooding.

"All we can do is warn people that (the water) is coming," he said. "It's really up to Mother Nature. And the water you guys are sending up here, if we don't have the ice gone by the time that happens, we have another big problem."

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