Rescuers rushed by land and air to evacuate people in Colorado stranded by epic mountain flooding as debris-filled rivers became muddy seas that extended into towns and farms miles from the Rockies.

Four people have been confirmed dead since the floods began on Wednesday and hundreds have not been heard from in the flood zone, which has grown to cover an area covering nearly 4,500 square miles, nearly the size of the US state of Connecticut.

Some of those who are unaccounted for may be stranded or injured and others might have got out but not yet contacted friends and relatives, officials said.

But police expected to find more bodies as the full scope of damage becomes clear.

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.

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.

Authorities warned residents who chose not to leave that they might not get another chance for a while and should be prepared to endure weeks without electricity, running water and basic supplies.

"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.

"I was thinking about staying. I could have lasted at least a year. I have a lot of training in wilderness survival," he said, adding that he probably had enough beer to last the whole time.

His wife, Meagan Harrington, said about 10 of their neighbours declined to evacuate. "They said they wouldn't force you, but it was strongly encouraged," she said.

Mr Shultz became tearful 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," he said.

Across the foothills, rescuers made progress against the floodwaters. But they were still unable to go up many narrow canyon roads that were either underwater or washed out.

Yesterday the surge of water reached the plains east of the mountains, cutting off more communities and diverting some rescue operations.

The military put more troops on the ground and helicopters in the air to aid the search-and-rescue effort. More rain was forecast.

By early today more than 1,750 people and 300 pets had been evacuated, Lt Col Mitch Utterback of the National Guard said.

A helicopter taking Colorado governor John Hickenlooper on a tour of the flooded areas stopped to pick up four stranded people and their two pets. The governor tweeted about the impromptu rescue, and spokesman Eric Brown confirmed it, but did not have any details.

Above the plains of Larimer County, rescue crews planned to fly as many missions as possible while skies were clear. Crews used inflatable boats to pick up families and pets from farmhouses. Some evacuees on horseback had to be escorted to safe ground.

Near Greeley, 35 miles east of the foothills, broad swathes of farmland had become lakes, and the raging South Platte and Poudre rivers surrounded more homes.

For those awaiting an airlift, Guardsmen dropped food, water and other supplies into the winding, narrow canyons. With supplies dwindling, residents of Lyons barbecued their food before it spoiled.

In neighboring New Mexico, state police reported the first death related to massive flooding this week from record heavy rains and overflowing rivers.

A man died after his vehicle washed into a ravine covered in mud near the Elephant Butte dam, probably on Friday during flooding, said Sgt Emmanuel Gutierrez of New Mexico State Police.