Boulders exceeding 100 tons crashed into a viewing area a half-mile up a popular day hike area, Chaffee County Undersheriff John Spezze said.
The slide left a football-field-sized gash below Mount Princeton, a 14,197-foot peak. A female hiker who heard the slide ran down the trail and called for help, Spezze said.
Rescuers found five dead bodies and a 13-year-old girl with a broken leg and other injuries. The girl was flown to a Denver hospital.
There was no immediate identification of the victims or whether they were a single group.
Chaffee County authorities dismissed an earlier report that there was a seventh hiker unaccounted for.
The slide wasn't preceded by smaller ones, Spezze said.
"It was totally unexpected. It caught everybody by surprise," he said.
Sheriff's department spokeswoman Monica Broaddus said rescuers left the mountain before dark Monday. She said the recovery effort would wait until likely Tuesday afternoon, after an engineer could survey the slide area to make sure it's safe to remove the bodies.
The slide occurred at about 11 a.m. on the trail to Agnes Vaille (VAYL) falls in the Pike and San Isabel National Forest, an easy day hike about a 2 1/2 hour drive southwest of Denver.
The trail is one of the first hikes recommended to people new to the area and is also popular with tourists, said Margaret Dean, a regular hiker who has hiked the trail with her 7-year-old grandson.
Dean, a copy assistant at The Mountain Mail newspaper in Salida, said the trail is easily accessible and provides a view of the falls and the Chalk Creek Valley in the Collegiate Peaks, which contains many mountains over 14,000-feet tall.
Agnes Vaille, the waterfall's namesake, was a Denver mountaineer who died in 1925 while attempting a difficult winter climb of Longs Peak, elevation 14,259 feet.