Gov. Brian Schweitzer warned that it could be a month before flooding dissipates in many areas of Montana.
Forecasters said Friday that heavy rains in the state over the holiday weekend followed by warming weather will likely speed up snowmelt and add to an inundation.
Authorities have already started releasing massive volumes of water from overburdened reservoirs.
The releases coupled with the floodwaters are predicted to move downstream and causing flooding in the Dakotas, and possibly in Iowa, Nebraska, Kansas and Missouri.
"There is going to be record flooding all along the Missouri River," said Paul Johnson, director of the Douglas County, Neb., Emergency Management Agency. "This isn't going to return to normal anytime soon."
Montana's rain swollen rivers and streams — 10 times their usual size in places — continued to carry a torrent of damaging debris through small towns and over roads Friday.
Cresting waters have broken through barriers and left hundreds of homes drenched in muddy floodwaters.
Eroded roads in rural areas left some residents stranded at their homes. State and local officials said they were making sure adequate food and water were delivered where necessary.
After authorities declared emergencies in 51 Montana counties, towns and Indian reservations, the governor toured some of the hardest-hit areas Friday.
"The people need help, not only in this area but all down the road. Some people lost everything," Kathryn Old Crow, 73, told Gov. Brian Schweitzer after his helicopter touched down near Lodge Grass, a remote Crow reservation town near the Wyoming border.
Hundreds were displaced after parts of the reservation got more than 8 inches of rain over just a few days this week. That's more than half of what the arid reservation receives on average over an entire year.
Memorial weekend rains will be hard on travellers, and not just because of bad weather. Dozens of roads and highways were closed around the state, including a 170-mile stretch of US-12 from Harlowton to Forsyth.
Officials warned that many campgrounds and fishing access sites also would be closed.
"We are going to be in floods not just here but all over Montana for the next 30 days," Schweitzer said during his tour of flooded areas.
And as all that water spills from high elevations toward the Great Plains, the U.S. Army Corps of Engineers said it has no choice but to continue unprecedented releases from brimming reservoirs. Dams built decades ago to contain dangerous floodwaters were now being used to release a carefully controlled torrent.
In Missouri and Nebraska, the river was already spilling over its banks and soaking low-lying farmland. More flooding was predicted as releases from swollen upstream reservoirs were expected to reach historic levels.
"We have gotten about a year's worth of rain in Montana in the last month," said Monique Farmer, spokeswoman for the corps' Omaha, Neb., district. "It's just crazy. It's been an unusual year."
Officials said water is encroaching on cities such as Fort Calhoun, Neb., and Sloan, Iowa — where residents built a temporary levee out of sandbags to help protect a dozen homes near the river.
The corps predicted 2011 could be one of the wettest years on record in the Missouri River basin, with flooding carrying on into July.
South Dakota officials decided to build emergency earthen levees on each side of the Missouri River in an attempt to protect homes in Pierre and Fort Pierre from floodwaters.
But South Dakota Gov. Dennis Daugaard said more releases from dams could flood some homes before work on the levee system can begin
"We believe the levee construction is certainly very good news, but some areas will be inundated even as the levees are being constructed," Daugaard said.
In Wyoming, Gov. Matt Mead said Friday he was deploying more than 100 Wyoming National Guard soldiers as more snow was predicted to fall this weekend and the state's huge mountain snowpack was expected to begin melting in earnest next week. Three Wyoming counties already have requested Guard help.
Schweitzer and U.S. Sen. Jon Tester toured waterlogged communities to gauge the damage. Several dozen homes in the central Montana town of Roundup remained underwater.
Roundup emergency officials said the closure of U.S. 12 cut off road access to the town from the west and east. The only way into Roundup by road was from the north because U.S. 87 south of town also was shut down.
On the Crow Reservation, 200 damaged homes had already been tallied. And Chairman Cedric Black Eagle expected that figure to rise.
Donations of food and other necessities were coming in to the tribe, but officials said more was needed. Also damaged by the high waters were irrigation systems, roads, bridges and other infrastructure.
"It will be six months to a year before things are back to normal," Black Eagle said.
About 300 reservation residents fled north to Billings to take temporary refuge on a college campus where the Red Cross set up a shelter. Crews worked Friday to restore full pressure to the reservation's water supply.
Associated Press writers Stephen Dockery in Helena, Josh Funk in Omaha, Neb., Chet Brokaw in Pierre, S.D., Bob Moen in Cheyenne, Wyo., and Maria Sudekum Fisher in Kansas City, Mo., contributed to this report.