Colorado has been hit with its worst floods in decades leaving four people dead and 172 people missing.

The flooding was so intense it toppled buildings in some places after the heavy rains began overnight on Wednesday.

The floods were triggered by unusually heavy late-summer storms that drenched Colorado's biggest urban centres, from Fort Collins near the Wyoming border, south through Boulder, Denver and Colorado Springs.

National Guard troops have plucked stranded residents out of danger by helicopter and rescued them from communities in military trucks.

Guard members went into the hard-hit town of Lyons through waist-high water and went door-to-door to pull out up to 2,000 trapped residents.

Elsewhere in the state, search and rescue teams used helicopters to hoist 200 residents to safety one-by-one.

Boulder and a string of other towns along the Front Range of the Rockies north of Denver were especially hard hit as water poured down rain-soaked mountains and spilled through canyons that funneled the run-off into populated areas.

Yesterday, Governor John Hickenlooper declared a disaster emergency for 14 counties, reaching from the Wyoming border south to Colorado Springs.