Four people have been confirmed dead since the harrowing floods began Wednesday. And hundreds of others have not been heard from in the flood zone, which has grown to cover an area covering nearly 4,500 square miles (11,655 square kilometres), nearly the size of the US state of Connecticut.
Some of those who are unaccounted for may be stranded or injured. Others might have gotten out but not yet contacted friends and relatives, officials said.
A woman was missing and presumed dead after witnesses saw floodwaters from the Big Thompson River destroy her home in the Cedar Cove area, 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," he said.
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National Guard helicopters and truck convoys broke through to paralysed canyon communities where thousands of stranded residents were eager to escape the Rocky Mountain foothills. But not everybody was willing to go. Dozens of people in the isolated community of Jamestown wanted to stay to watch over their homes.
"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," Boulder County Sheriff Joe Pelle said.
Special education teacher Brian Shultz, 38, was torn about leaving his Jamestown home as he sat outside a makeshift shelter at a high school.
His wife, Meagan Harrington, gave him a wry smile. About 10 of their neighbours declined to evacuate, she said.
"They said they wouldn't force you, but it was strongly encouraged," she said.
Shultz teared up behind his sunglasses as he compared his situation to that of his neighbours.
"At least all of our stuff's there and will be there when we get back. The people right by the river, their houses were washed away. Other people thought their houses were going to be OK, and then they started to go. It's just really devastating."