(CNN) -- Authorities in Portage, Wisconsin, worked Sunday to evacuate residents as a levee along the Wisconsin River approached imminent failure after heavy rainfall soaked the Midwest last week.
"It could collapse at any time," said Greg Matthews, a spokesman for Wisconsin's Department of Natural Resources.
An alert sent out by Columbia County Emergency Management on Sunday urged residents near Blackhawk Park to evacuate immediately ahead of the flooding, which is expected to wash out a main road leading to about 150 residences.
"Emergency vehicles including police, fire and EMS will not be able to reach residents," the statement said.
By 4 p.m. (5 p.m. ET), it was unclear how many residents remained in their homes, said Kathy Johnson, the deputy director of the county's emergency management department.
"Anyone in there right now won't be able to come out," she told CNN.
A Red Cross shelter was opened at a nearby church to accommodate displaced residents.
The Wisconsin River crested Sunday at 20.4 feet -- nearly 3.5 feet above flood level, according to the National Weather Service, putting major pressure on the 120-year-old Caledonia-Lewiston levee system.
Officials with the Department of Natural Resources have been monitoring the levee all week and working to repair any problem areas, but Matthews said Sunday that "this is one problem we have not been able to contain and it's getting worse."
The levee system, built in the 1890s, was constructed from locally available materials -- mostly sand -- "without any engineering design or adherence to any standards," DNR said in a statement Friday.
"This is a 120-year-old relic," Matthews said. "It's made of sand. ... A modern levee that our engineers would be familiar with would be constructed of steel and concrete. So if it does fail we really quite frankly don't know the extent and scope of what might happen."
The river is expected to begin gradually falling Sunday evening before dropping below flood stage by Wednesday, the weather service said.
The levee's anticipated failure comes after a week of rainfall that dumped as much as 11 inches of rain in parts of Wisconsin and neighboring Minnesota.
Floodgates along the Menominee River in Niagara, Wisconsin, were opened earlier this week to ease pressure on dams. Video showed a torrent of water gushing from the overflowing dam.
"I have never seen the water rage down like it is," CNN iReporter Jason Asselin said.
In Northfield, Minnesota, authorities called late last week for large scale sandbagging in an effort to protect homes and businesses from the rising Cannon River Canal, according to iReporter Peter Holland.
And in Biron, Wisconsin, just north of Wisconsin Rapids in the central portion of the state, iReporter Tim Krause said Saturday that the flooding is "starting to get a bit scary. We have houses that are flooding in our neighborhood."
CNN meteorologist Jacqui Jeras said Sunday that there's no immediate end in sight to the flooding.
"Even though some of these rivers have crested ... keep in mind that they're still in flood, so this is going to be a problem for a couple of days," Jeras said. "Many of these rivers are all going to be dumping into the Mississippi River which then in turn will rise up and we'll see some flooding there, maybe in La Crosse and the Winona areas later in the week."
CNN's Don Lemon contributed to this report.