Rescuers rushed by land and air today to evacuate Coloradoans stranded by epic mountain flooding, as debris-filled rivers became muddy seas that extended into towns and farms far from the Rocky Mountains.

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 11,655 square kilometres, twice the size of Prince Edward Island.

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. 

Two of the dead were identified by the Boulder County coroner as a couple, both 19, who authorities believe died when they were swept away after driving into floodwaters and then leaving their vehicle.

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

By Saturday night, 1,750 people and 300 pets had been evacuated from Boulder and Larimer County, the National Guard said.​

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.

'They are threatening us'

"Essentially, what they were threatening us with is, 'If you stay here, you may be here for a month,"' said 79-year-old Dean Hollenbaugh, who was evacuated by helicopter from Jamestown, northwest of Boulder.

For those awaiting an airlift, the National Guard dropped food, water and other supplies in Jamestown and other small towns in the winding, narrow canyons that dot the Rocky Mountain foothills.

Colorado Flooding

A road north of Boulder, Colo., was washed out by the overflow of a creek. (Matthew Kennedy/Earth Vision Trust/AP)

Thousands of evacuees sought shelter in cities that were nearly surrounded by raging rivers spilling over their banks.

One was Mary Hemme, 62, who displayed a pair of purple socks as she sat outside a church in Longmont. They're a memento of the more than 30 hours she spent in an elementary school in the flood-stricken mountain town of Lyons. Many evacuees — eventually rescued by National Guard trucks — got socks because most of them had wet feet, Hemme said.

She recalled the sirens blared at 2:30 a.m. MT Wednesday.

"Mary we have to go, this place is flooding," she recalled her friend Kristen Vincent saying as they clambered out of a trailer.

'It was rushing at us'

"And we stepped out of the trailer, onto the ground where the cars were parked, and it already like this, almost to our knees," she said. "It wasn't just sitting there. It was rushing at us."

Soon the trailer, like others in the park where she was staying, was submerged.

Hemme said she walked up at hill a daybreak and surveyed the trailer park.

"The most terrifying thing was when I climbed up on that cliff and looked down. It was the meanest, most — I mean, no wonder it carries cars like toys," Hemme said. "I was so afraid that I was going to die, that water came so fast."

Colorado Flooding

This aerial photo shows a populated area near Longmont, Colo., being overrun by water. (Dennis Pierce/Colorado Heli-Ops/AP)

On Saturday, the surge of water reached the plains east of the Rocky 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 in the search-and-rescue effort. More rain was in the forecast.

By midday Saturday, nearly 800 people and their pets had been evacuated from at-risk locations, National Guard Master Sgt. Cheresa Theiral said. More than 700 people spent Friday night in shelters, according to the Red Cross.

Terry Kishiyma's son flagged down a helicopter with his shirt after a three-day wait for rescue from a neighbor's house on higher ground.

"You could hear the choppers for miles and miles, but I didn't know if they were evacuating people. You see a chopper going down behind a ridge, and you have no clue," Kishiyma said.

In addition to his son's efforts, Kishiyma said his wife shouted at the chopper, "We have babies!"

Bridges washed away

It will be weeks, if not months, before a semblance of normalcy returns to Lyons, a gateway community to Rocky Mountain National Park. The town, surrounded by sandstone cliffs whose colour was reflected in the raging St. Vrain River, consisted of six islands Friday as residents barbecued their food before it spoiled. Several people set up a tent camp on a hill.

Some 2,500 residents were being evacuated from Lyons. Two bridges that led into the area were washed away. 

Boulder County Sheriff Joe Pelle said recovery would be long and expensive — similar to wildfires the state is more familiar with.

"Please be patient. This is an unprecedented event," Pelle said.