"This is a 500-to-1,000-year flood," Sean Conway, a commissioner of rural Weld County, said at a news conference.
The National Guard called Saturday's rescue efforts to save those stranded by the floods the biggest helicopter resuce effort since Hurricane Katrina.
"This is clearly going to be a historic event," National Weather Service Director Louis Uccellini told Climate Central's Andrew Freedman. "The true magnitude is really just becoming obvious now."
ITN has video:
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