Minot North Dakota

Sirens are blaring at this moment in Minot, N.D., as the overflowing Souris River floods over the top of local levees five hours before the evacuation deadline for 11,000 residents. Farther south, the overflowing Missouri River has put two nuclear power plants at risk, necessitated evacuations and produced a travel nightmare as interstate highways shut down.

"What I see right now is probably the most devastating in terms of the number of people directly impacted and what will likely be the damage to homes as the water begins to overtop the levees and fill in behind," National Guard Cmdr. Dave Sprynczynatyk said today.

Nearly 500 North Dakota National Guard soldiers are in the town of 41,000 people to help the last stragglers in the affected area get out of harm's way. They are accompanying the roaring sirens with shouts of "All residents must evacuate!"

"We've never seen anything like what we're expecting," Minot Mayor Curt Zimbleman told ABC News. The mayor had warned residents previously today that the river could top the levees earlier than expected, and has been urging residents to leave potentially affected areas.

Minot is expecting the worst flooding it has seen in nearly four decades, when severe flooding of the Souris River devastated the city in 1969. The same river reached 1,555.4 feet above sea level during that destructive flood time, and this time it could reach 1,563 feet.

For the second time in as many months, the Cass County Sheriff's Department airboat squads have been deployed to western North Dakota.

"The dedication of these people is amazing. When the call came out, the sheriff contacted us yesterday and the guys were pretty eager and ready to do their part. That's why we're here," Sgt. Duane Nitschke of the Cass County Sheriff's Department told ABC News Fargo affiliate WDAY.

The U.S. Nuclear Regulatory Commission is closely watching conditions along the Missouri River where floodwaters are rising at Cooper Nuclear Station and the Fort Calhoun nuclear power plant in Nebraska.

The Fort Calhoun plant was shut down on April 7 for a refueling outage, and operators decided not to restart it until flooding had subsided. The Cooper plant was shut down for an "unusual event" on June 19.

"Both plants have activated their flood response plans and taken appropriate steps to protect vital structures, systems and components from rising floodwaters and maintain their plants in a safe condition," NRC Region IV Administrator Elmo Collins said in a statement today.

Although the Fort Calhoun plant is surrounded by an eight foot tall and 16 foot wide protective berm, two feet of water have already made its way to several areas of the Fort Calhoun plant, but authorities say there is no immediate danger at either plant.

Many residents of affected areas are having difficulty traveling, as Interstate 29 has been shut down in the region and every bridge crossing from St. Joseph, Mo., to Omaha, Neb., has been closed.

In Holt County, Mo., 600 residents of the towns of Corning and Fortescue have been evacuated, as authorities say that sustained flooding could keep those residents out of their homes for several weeks.