Overland flooding in the Red River Valley spread to previously unseen lengths and depths Monday, isolating hundreds of rural homesteads and closing long stretches of roads on both sides of the Minnesota-North Dakota border.

High water in the Red River, which crested at Fargo-Moorhead Saturday, has been causing tributaries to back up and flow across the region's flat landscape. National Weather Service hydrologist Steve Buan said that is likely to continue as the Red's crest rolls northward to Canada.

"It's totally new, these kinds of depths of water," said Clay County Sheriff Bill Bergquist, who's lived his whole life in the Fargo-Moorhead area. "It's become more of an emergency, because a lot of these people didn't pre-plan. Some of them never had to sandbag before."

The extended flooding has closed a 40-mile stretch of Interstate 29, which parallels the border between the states. On the Minnesota side, 100 miles of roads are closed in Clay County.

The Minnesota Department of Natural Resources sent an airboat to Moorhead as part of an effort to contact residents surrounded by water whom county officials have been unable to reach in recent days. No one had asked to be evacuated, but officials across the region, who have been joined by emergency crews and equipment from Minnesota, North Dakota and federal governments, have been on guard for medical emergencies, failed dikes around homes and other crises.

The U.S. Army Corps of Engineers was raising a permanent dike around Hendrum, Minn., by 2 extra feet Monday to help hold off water from the nearby Wild Rice River, which was rising rapidly. Nearly 300 Minnesota National Guard soldiers are patrolling levees along the Red River.

Small cities see worst

The U.S. Geological Survey measured a record flow Sunday on the Sheyenne River, just north of Fargo. Small cities in that area are experiencing worse flooding than they did during the record floods of 1997 and 2009, the Weather Service noted.

The crest at Fargo-Moorhead was the fourth-highest ever. The river was down about 6 inches Monday afternoon and is expected to drop a little more than 2 feet over the next week. But the Weather Service has raised the predicted crest for Grand Forks and East Grand Forks, Minn., by a foot because of heavy rain Sunday feeding the Red Lake River, a key tributary. That crest, expected Friday, would be second only to the devastating flood of 1997, but 8 feet below the level of permanent levees built since then.

Buan noted that in the extremely flat Red River Valley, human imprint on the landscape -- raising a road, building some housing, and even building dikes -- can accelerate overland flooding. "Inches make a difference," he said. "You raise a road, it could impact somebody by a foot."

Water was running 8 inches deep across Interstate 29, from north of Fargo to Hillsboro, and state officials said it could remain closed for several more days. To the east in Minnesota, Hwy. 75 was closed in several spots between Moorhead and East Grand Forks, as was the Demers Avenue bridge between the downtowns of East Grand Forks and Grand Forks, N.D.

Fargo spent between $6 million and $8 million preparing for this year's flood and again relied on work from thousands of volunteers. Both Fargo and Moorhead also have spent tens of millions buying and clearing away houses and building permanent levees. Dodging a calamity again prompted Mayor Dennis Walaker to emphasize Monday the need for a planned $1.7 billion flood diversion project.

"One of these days we're going to lose," he said. "We need to keep moving this thing forward. Three floods in a row -- they're sick of sandbagging."

High water also was creating problems far from the Red River.

The Corps of Engineers closed the lock and dam on the Mississippi River at Alma, Wis., due to debris and high water, effectively shutting down commercial barge traffic for several days. In the metro area, the Stillwater Lift Bridge remained closed. But flood pressure was easing in St. Paul, South St. Paul and Scott County.

Judson Freed, director of Ramsey County emergency management, expects the Mississippi River to crest and start to go down midweek. "Barring more rain, it will go down," he said. "But the river is still sensitive."

The water still lapped close to the Harriet Island boathouse Monday, meaning the city-controlled park isn't likely to open soon. After the water does go down, there is likely to be lots of debris to clean up.

On whether the levees will come down soon: "Not this week, but not much longer. We'll go by what the river says," Freed said. "The weather really helped us out a lot; it's kind of nice for a change."

Staff writers Rochelle Olson and Paul Walsh contributed to this report. Bill McAuliffe • 612-673-7646