(Adds details from latest Army Corps briefing; flood details)
By Karen Dillon
KANSAS CITY, Mo., March 21 (Reuters) - Floodwaters that devastated swaths of Nebraska and Iowa rolled downstream along America's longest river on Thursday, swamping more Midwestern farmland as waterfront communities in Missouri and Kansas hurried to shore up strained levees.
Flooding of the Missouri River triggered by last week's so-called "bomb cyclone" storm has already inflicted damage estimated at nearly $1.5 billion in Nebraska, killed at least four people in Nebraska and Iowa and left a man missing below Nebraska's collapsed Spencer Dam.
Missouri Governor Mike Parson declared a state of emergency for his state as high water forced evacuations of several small farm communities. Larger towns from St. Joseph to Kansas City braced for additional flooding forecast through the weekend.
"The rising floodwaters are affecting more Missouri communities and farms, closing more roads and threatening levees, water treatment plants and other critical infrastructure," Parson said in a statement.
The declaration allows state resources and assistance to be provided directly to counties and municipalities in need, said Mike O'Connell, spokesman for the Missouri Public Safety Department.
Authorities say continued flooding in the days ahead is unlikely to reach the widespread, catastrophic scale seen in Nebraska and Iowa - as excess flow dissipates along the length of the river and water breaches or flows over the tops of levees.
But the threat of extensive flooding lingers over the wider region through May and could grow dire in coming weeks with additional rainfall and melting snow runoff, National Oceanic and Atmospheric Administration officials said on Thursday.
"This is shaping up to be a potentially unprecedented flood season, with more than 200 million people at risk," Ed Clark, director of NOAA's National Water Center in Tuscaloosa, Alabama, said on Thursday in the agency's spring outlook.
Scientists said on Thursday that climate change played a hand in the deadly floods, while a Trump administration official said more study was needed before making that link.
LEVEE BREACHES
Floodwaters have already swamped large stretches of Missouri, Nebraska and Iowa, drowning livestock and damaging crop land along the Missouri. A state of emergency has been declared in all or parts of the three Midwestern farm states.
The river's next major flood crest is forecast to hit St. Joseph, Missouri, early Friday morning and a day later in Kansas City, Missouri, 55 miles (90 km) to the south.