“This is usually just a little stream that you wouldn’t even think to fish in,” Ms. Reid said.

The flooded highway has cut off residential areas for the foreseeable future, sweeping up whole willow and cottonwood trees as if they were twigs. John and Billie Brumder, who live across from the Crestview Estates neighborhood, said they were awakened at about 1:30 a.m. Thursday by the frightening sound of large rocks being dragged along the creek bed.

“When I heard that, I knew it was a big deal,” he said. “I knew we had to get out of there.”

Their car, which was already partly submerged, stalled, Mr. Brumder said, but they were able to escape in their pickup truck.

The authorities said Thursday afternoon that they expected the death toll in the state, currently at three, to rise.

One person drowned and a second person was killed when a structure collapsed in the town of Jamestown, northwest of Boulder. The third victim was found in Colorado Springs by police officers conducting flood patrols.

Rain had been falling in the Boulder area since Monday, but picked up significantly on Wednesday evening, causing mudslides in Colorado’s Front Range, where since 2010 wildfires have denuded some areas of trees and brush that would have normally helped soak up the moisture.