Deadly flash floods in Colorado 'not an ordinary disaster'

Three vehicles sit in water after crashing where a road collapsed following flooding at Highway 287 and Dillon at the Broomfield/Lafayette border, Colorado. Photo / AP

Heavy rains sent walls of water crashing down mountainsides in Colorado, cutting off remote towns, forcing the state's largest university to close and leaving at least three people dead across a rugged landscape that included areas blackened by recent wildfires.

A storm system has been dropping rain on the region for much of the week. Up to 20 centimetres fell in an area spanning from the Wyoming border south to the foothills west of Denver.

Flooding extended all along the Front Range mountains, including the cities of Colorado Springs, Denver, Fort Collins, Greeley, Aurora and Boulder.

Numerous roads were washed out or made impassable by floods. Parts of several interstate highways in the Denver area were closed for a time. Floodwaters poured into homes, and at least a few buildings collapsed in the torrent.

Boulder County appeared to be hardest hit. Sheriff Joe Pelle said the town of Lyons was completely cut off because of flooded roads, and residents were huddling together on higher ground. Although everyone was believed to be safe, the deluge was expected to continue into Friday.

"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."

Rain is normally soaked up by a sponge-like layer of pine needles and twigs on the forest floor.

But wildfires incinerate that layer and leave a residue in the top layer of soil that sheds water.

A relatively light rain can rush down charred hillsides into streambeds, picking up dirt, ash, rocks and tree limbs along the way. Narrow canyons aggravate the threat.

- AP