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TIM NELSON: If you're standing on the edge of the Red River these days and wondering where all that water came from, think back to last summer. In North Dakota, it may have been one of the wettest Augusts in 250 years, with more than twice the precipitation than average.

Almost four inches of rain fell the second week of that month in Fargo alone-- and it kept up. 5 inches in September and 4 and 1/2 in October. That's twice the rain the area gets in April. A freak rainstorm swept through the area in late November to make matters worse.

Steve Boone is a coordinating hydrologist with the National Weather Service in Chanhassen, which is also the home of the River Forecast Center, which charts river levels around the upper Midwest. He's going to walk us through what happened starting six months ago.

STEVE BOONE: What we saw was that our soil moisture accounting models were basically brim full when showing of saturated soil condition at freeze up.

TIM NELSON: That happened the second week of December, when the temperature fell by about 50 degrees from the start to the middle of the month. The landscape froze like a big wet sponge, locking the moisture in place. And by mid-winter, US Geological Survey crews were seeing the signs of trouble coming.

STEVE BOONE: What we saw this winter was extremely high flows in all the tributary rivers that we're feeding into the red. They were at-- some of them had record high flows all winter long. And that indicates that the soil profile was full. It could not take any more.

TIM NELSON: On February 23, Boone and another hydrologist hit the road, meeting with dozens of officials and telling them that serious flooding was heading for the Red River Valley. But they weren't sure exactly how much. Boone had seen floods before. He worked on the forecast for the 1997 floods, and everyone remembered the 10 feet of snow that fell that winter.

STEVE BOONE: It was a very visual experience.

TIM NELSON: But it's the water you don't see that matters.

STEVE BOONE: When it comes to total amounts of moisture, this winter far exceeds-- this fall and winter far exceeds the 1996/97 period for total amounts of moisture.

TIM NELSON: And that's where things get complicated. Think of it this way. Hydrologists measure river flow in cubic feet per second.

STEVE BOONE: A cubic foot is about the size of a basketball. So if you hear them saying 30,000 cubic feet per second going through Fargo visualize about 30,000 basketballs per second going by.

TIM NELSON: It's easy enough for forecasters to see that volume of water coming, but what's tougher is knowing how high and how far out that water will spread. The Red River Valley is amazingly flat in geographic terms, and it makes it very difficult to figure out exactly where the river will end up for any given amount of water.

STEVE BOONE: Nature likes a gradient of-- likes a fairly high gradient. It likes several feet per mile to push water down, and the Red River Valley is about one foot per mile. So there's a lot of complicating factors that what we call multi-dimensional flow.

TIM NELSON: And in layman's terms.

STEVE BOONE: In the Red River Valley, water likes to go sideways.

TIM NELSON: So when the water reaches record volumes and history is no longer a guide, forecasters have to put aside the maps and start converting the amount of water, known as discharge, into the depth of water at any given spot, known as a stage.

STEVE BOONE: What you're describing there is what we call hydraulic analysis, which uses a lot of mathematics, calculus, differential equations, things that are very hard even for the college graduate to comprehend-- you need a master's degree. And that has to take into account all what we call the hydraulic characteristics.

That would be the channel roughness, how many objects are in the river channel or the floodway as the water is going through? How many bridges are in there? Any other objects that are in the channel? Ice, things like that, all have to be mathematically taken into account.

TIM NELSON: Forecasters started on that homework earlier this month as the weather warmed up. They came up with their initial estimates for how deep the river would get somewhere in the upper 30 foot range, as measured in Fargo.

But as they zeroed in on their forecast last weekend, clouds gathered and the targets started moving. Almost 3 inches of precipitation fell in four days as rain and snow. Then temperatures plunged. That meant more water, more ice, maybe more trees in the river as the water rose, maybe more bridges in the way. It was a different river.

STEVE BOONE: That makes all these analyses much more difficult.

TIM NELSON: When the numbers were finally crunched, the best guess came out Thursday night. The estimate only moved up a couple of feet, a mathematically small proportion of the overall depth of the river. But in Fargo these days, every inch matters. Tim Nelson, Minnesota Public Radio News.