Stick a river that flows north into one of the flattest stretches of land on the face of the Earth and what do you get?
That is why folks in the Red River Valley seem to spend spring after spring stacking sandbags, watching the skies and worrying about frost depth, rate of melt and ice jams.
In a region that slopes inches per mile, all it takes is a rapid thaw or an ice dam to send the meandering Red River over its banks and put miles of the valley under water.
"It's like taking a glass of water and pouring it on the top of a table," said Kevin Dean, public information officer for the city of Grand Forks, N.D.
"The water just goes everywhere. Everything's flat. That's really hard for a lot of people to understand. But it's just so flat that when it floods, it has everywhere to go and there's nothing to stop it."
It's been that way for nearly 10,000 years, since glacial ice last pushed south across parts of North and South Dakota and Minnesota, creating a "very gentle bowl shape" where the states meet, said Adam Lewis, an assistant professor of geoscience at North Dakota State University in Fargo.
When the ice pulled back, a massive lake, Glacial Lake Agassiz, stretched from central Minnesota north into Canada.
As mud and sediment settled, it smoothed over the lake bottom. Later, after the lake drained, it left behind the "rich, black, fabulous soil" of the Red River Valley, which boasts some of the most fertile land in the region, Lewis said.
Yet because the basin was so flat, its rivers never developed the energy to cut deep into the landscape.
From Breckenridge, Minn., to the Canadian border, about 200 miles north, the valley drops about one foot per mile.
"There's just not enough energy in this river system to cut a good channel," Lewis said. "So what you end up with are these broad, meandering streams that move very slowly and very lazily across this landscape."
Because the topography changes little, "It's a rare year that we don't have a flood," said Mark Ewens, data acquisition manager for the National Weather Service in Grand Forks.
The northern flow of the river doesn't help, either.
"Typically, [the river] opens up in the south end first and starts to move while the northern half is frozen," Ewens said. "And the water immediately starts running into ice jams."
The severity of the flooding depends on a combination of other factors, too, including how much moisture is in the soil before freezing, the depth of the frost, the depth and moisture content of the snow pack and the speed of the thaw.
A rapid thaw almost certainly means trouble, while a slow, gradual thaw and freeze cycle slows runoff.
"Every year is different," Ewens said.
A heavy, wet snow pack and a rapid melt in 1997 were primary reasons for the flooding that swamped Breckenridge, Grand Forks and East Grand Forks that spring.
The primary problem this year has been the heavy fall rains that saturated the soil, and the freezing of the water in the soil as winter approached.
"The very wet fall is a double-edged sword," Ewens said. "Not only does it saturate the soil, it allows the soil to freeze deeper because wet soil freezes deeper," meaning the snow melt runs off more quickly because it can't penetrate the soil.
The rapid thaw of the past week or so hasn't helped.
"This is an ancient glacial lake," Lewis said. "And if you get too much water, the lake just refills."
Richard Meryhew • 612-673-4425