(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 7 million people were under flood warnings Tuesday, 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.

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."

A portion of the Offutt Air Force Base in Bellevue, Nebraska was heavily flooded on Tuesday.

Vice President Mike Pence arrived Tuesday in Nebraska, where he is set to tour areas devastated by record-breaking flooding and visit a shelter where he'll meet residents displaced by the flooding.

"We are going to stand with you," Pence told reporters. "We are going to make sure that the federal resources are available for these families, these communities and your states to rebuild bigger and better than ever before."

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

Visited volunteers, first responders, and local officials in Wood River this afternoon to see the flood impact in the community — this neighborhood street has been turned into a small river.#NebraskaFlood | #NebraskaStrong pic.twitter.com/7EkpT8Yzba — Gov. Pete Ricketts (@GovRicketts) March 19, 2019

Ricketts and Iowa Gov. Kim Reynolds met with Pence in Nebraska and asked him how federal funding can be expedited. Iowa has declared 41 of its 99 counties as disaster areas.

Earlier, Reynolds said she saw catastrophic damage in the western part of her state during a flyover a day earlier. Though numbers still are coming in, state officials are hearing farmers lost significant amounts of stored grain and livestock, she said.

"It looked like an ocean," Reynolds told reporters at the state Capitol in Des Moines. "I mean, I saw the top of grain bins. We saw buildings flooded. It's just unbelievable, and that's people's lives. Those are fifth-generation farms. Those are businesses, communities."

Impact in damages in the millions

Lincoln, Nebraska, Mayor Chris Beutler requested a disaster declaration from the governor on Monday 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, saying 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."

In the small Nebraska community of Archer, water from flooded creeks was nearly filling up the basement of Bruce and Maria Garretson's home on Monday, CNN affiliate KHGI in Kearney reported.

"Well, this is our new swimming pool," he told KHGI, with the water nearly reaching the top of the stairs. The couple said they had little choice but to let the basement be.

"There's no floor down there. It's all just sand and dirt, and if we pump the water out completely, it will take all that sand out that's supporting our foundation, so we have to leave it," Maria Garretson told the TV station.

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."

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3 - Donate Blood or Platelets visit - https://t.co/IkCAIKZMHw pic.twitter.com/aAxjL9ulV4 — Nebr. / SW Iowa Red Cross (@neiaredcross) March 19, 2019

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.

An aerial view shows flooding Sunday at Camp Ashland, Nebraska, after a levee broke.

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.