Officials are warning families in Boulder, Colorado, to evacuate immediately as severe floods have seen four people die and left 500 unaccounted for.
Days of heavy rain has forced the rescue of more than 2,500 people as more than 15in of rain - nearly half the region's annual average - has called in one week, reports the BBC.
Thousands of people have been warned to leave their homes, with Boulder County Sheriff Joe Pelle warning that the National Guard helicopters and truck convoys would not go back for people who refuse to leave.
He told Sky News: "We're not trying to force anyone from their home. We're not trying to be forceful, but we're trying to be very factual and definitive about the consequences of their decision, and we hope that they will come down."
According to The Sun, Governor John Hickenlooper declared a disaster emergency for 14 counties, reaching from the Wyoming border south to Colorado Springs.
The declaration authorised $6million (£3.8million) to pay for flood response and recovery.
And President Barack Obama has even approved federal disaster aid for Boulder County.
The flood zone has now grown to an area of 4,500 square miles. Rescue crew have used inflatable boats to pick up families and pets from farmhouses, while those awaiting airlift in narrow canyons have been dropped food and supplies.
Boulder County Sheriff Joe Pelle told USA Today: "It is a sinking feeling when you realise that when some people call ... we are not going to be able to get to them. But we are making great progress."
The Boulder county coroner identified two fatalities as Wesley Quinlan and Wiyanna Nelson, both 19, who are believed to have been swept away after driving into floodwaters and leaving their vehicle.
At least two others have died but Larimer County sheriff's spokesman John Schulz said: "I expect that we're going to continue to receive reports of confirmed missing and confirmed fatalities throughout the next several days."
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