Floods in Colorado have killed three people and prompted the evacuation of thousands others.

Heavy rains have been falling for much of the week, swelling creeks to dangerous levels and sending walls of water crashing down mountainsides.

President Barack Obama signed an emergency declaration releasing federal aid and allowing the Federal Emergency Management Agency to coordinate disaster relief efforts.

About 8,000 people were ordered to evacuate in the Boulder County area, north of Denver, the worst hit by the storm, where at least two deaths occurred.

One person died after being crushed under a collapsed structure in Jamestown, while a second drowned as he was trying to help a woman who had been swept away in a torrent of water. The woman is still missing.

Floods caused the closure of many roads, cutting off remote areas and preventing rescue crews from reaching the stranded.

Jamestown, Lyons and other towns in the Rocky Mountain foothills have been isolated and without power or telephone.

Sheriff Joe Pelle said Lyons was completely cut off because of flooded roads, and residents were huddling together on higher ground.

"It is not an ordinary disaster," Pelle said. "All the preparation in the world ... it can't put people up those canyons while these walls of water are coming down."

"There's no way out of town. There's no way into town. So, basically, now we're just on an island," said local resident Jason Stillman.

The state's largest university, the University of Colorado, remained closed and about 400 students were evacuated.

"Wall of water coming down Boulder Canyon. Stay away," a message posted on the university's Web site read, advising students to stay indoors.

Rain poured over an area spanning from the northern Wyoming border to the foothills west of Denver, affecting the state capital and other cities including cities of Colorado Springs, Fort Collins, Greeley and Aurora.

In Colorado Springs police found the body of a 54-year-old man in a creek on the west side of the city.