By Christina Maxouris, CNN

(CNN) -- The major flooding that left at least four dead in Nebraska and Iowa over the past week is slowly receding in some areas, but rivers likely will be flooded for days more, and additional rain and melting snow are expected to make flooding worse in other parts of the Plains and Midwest.

More than 8 million people were under flood warnings Tuesday morning, the National Weather Service said, as the heartland deals with the aftermath of snow melt and a "bomb cyclone" that blasted the central Plains and Upper Midwest last week with heavy rain or snow.

Nebraska, Iowa and Wisconsin have declared states of emergency.

The flooding in Nebraska has been especially widespread, with Gov. Pete Ricketts calling it the "most widespread disaster we have had in our state's history."

Vice President Mike Pence is expected to travel Tuesday to Nebraska to tour areas devastated by record-breaking flooding.

Flood records have been shattered in 17 places across the state, the Nebraska Emergency Management Agency said. Seventy-four cities, 65 counties and four tribal areas have issued emergency declarations in the Cornhusker State.

Impact in damages in the millions

On Monday, Lincoln Mayor Chris Beutler requested a disaster declaration from the governor after flooding in a city well field caused power outages, spurring officials to ask 284,000 residents to restrict their water usage.

Bryan Tuma, assistant director of the Nebraska Emergency Management Agency, said the Red Cross is so far operating nine shelters, which are housing more than 470 people. Twenty-nine counties had mandatory or voluntary evacuations, 18 of which have now allowed their residents to return.

The Nebraska Department of Health and Human Services also warned the flooding may have contaminated private water supplies, adding wells should be tested to ensure safety.

"Cloudiness or a change in taste or smell are signs of possible contamination," said Sue Dempsey, administrator of the department's Drinking Water Program. "If there is any indication that the water supply has been breached by floodwaters, even without noticeable changes in taste or smell, I encourage residents to get a water sample kit for testing."

Meanwhile, Nebraska has already seen a $205 million impact on its public sector from the floods, the emergency management agency said. The state's private sector has seen a $60 million impact.

Ricketts said Monday the state has been working with the Federal Emergency Management Agency and other officials "to get emergency disaster declaration declared, and tap into those federal resources for public assistance and individual assistance."

Flooding could worsen in parts of the heartland

Two major factors led to the heavy flooding: a bomb cyclone that dropped heavy rain from March 13 into late last week, and melting ice and snow. The rain, in some cases, dropped onto snowpack and frozen ground that weren't able to absorb the downpour.

The rain and melting snow overwhelmed rivers and streams.

Water levels were dropping in Nebraska and Iowa on Tuesday, but flooding may get worse elsewhere in the coming days.

Significant snow cover remains in North and South Dakota and Minnesota, and melts are likely to cause flooding in the Dakotas into next week.

The Missouri and Mississippi rivers south of Nebraska and Iowa -- already at minor or moderate flood stage along Kansas, Missouri and Illinois -- are expected to rise a few more feet over the next few days, the National Weather Service said.

At least 4 flood-related deaths in 2 states

At least four people died as a result of flooding last week in Nebraska and Iowa, authorities said.

James Wilke, a farmer in Nebraska's Platte County, died after trying to respond to a call to help rescue a person trapped in a vehicle, Nebraska Emergency Management Agency spokesman Mike Wight said.

"With the guidance of emergency responders, James drove his tractor over the Shell Creek bridge on the Monestary Road and the bridge gave out," family friend Jodi L. Hefti wrote on Facebook. "James and the tractor went down into the floodwater below."

Another Nebraska man died after floodwaters near a dam in Spencer overwhelmed him, Wight said.

Near Columbus, Nebraska, Betty Hamernik, 80, died after floodwaters trapped her in her home last week, the Platte County Sheriff's Department said. Rescuers tried to reach her home for two days before they found her dead early Friday. Hamernik's cause of death remains under investigation.

In Iowa, Nebraskan Aleido Rojas Galan was one of three people rescued from floodwaters, the Fremont County Sheriff's Office said. But Galan, 55, succumbed to his injuries and died at a hospital in Lincoln.