LYONS, Colorado (AP) -- Thousands more people in the inundated Colorado city of Boulder were ordered to evacuate as water rose to dangerous levels amid a storm system that has been dropping rain for a week.

Rescuers struggled to reach dozens of people cut off by flooding in mountain communities, while residents in the Denver area and other areas were warned to stay off flooded streets.

At least three people were killed and another was missing.

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

Some towns in the Rocky Mountain foothills have been isolated by flooding and without power or telephone.

Late Thursday, Boulder city officials said they sent a notice to head to higher ground to about 4,000 people living along Boulder Creek, according to a report in Boulder's Daily Camera newspaper.

Boulder County spokesman James Burrus told The Associated Press that about 8,000 telephone numbers with the message to evacuate were called.

"There's no way out of town. There's no way into town. So, basically, now we're just on an island," said Jason Stillman, 37, who was forced with his fiancee to evacuate their home after a nearby river began to overflow into the street.

One person was killed when a structure collapsed. Another man drowned in flood waters, and a woman who was with him was missing after swept away when the vehicle she was riding in got stuck.

The man died after getting out of the vehicle to help her, Commander Heidi Prentup of the Boulder Sheriff's Office said.