An Idaho Springs police spokesman said the unidentified man's body was recovered from Clear Creek about 3 miles downstream from where he went into the water.
The Denver Post reported the state Emergency Management Office said there were three other deaths in Boulder County, two in El Paso County and two missing and presumed dead in Larimer County.
The body of another unidentified victim was recovered by authorities from the West Fork of Sand Creek in Colorado Springs Monday.
The Post said by Monday afternoon there were still 183 people unaccounted for in Boulder County and 260 in Larimer County.
State emergency management officials estimated nearly 17,500 homes had been damaged and 1,500 destroyed along a 200-mile stretch of Colorado's Front Range.
Nearly 12,000 people were evacuated from their homes.
The floods also left 15 Colorado National Guard members and other emergency workers stranded on high ground near Lyons Monday. They had been evacuating residents Sunday night but were forced to abandon their mission when floodwaters began to rise, CNN reported.
A U.S. Army helicopter was able to rescue the residents and some of the troops and emergency workers before the weather became impassible.
The confirmed or presumed dead also included an 80-year-old woman and a 60-year-old woman who disappeared in raging floodwaters, a Larimer County Sheriff's Department spokesman in Fort Collins said.
The older woman "was injured and couldn't get out of her home," John Schulz said. "When people came back to help her, the house was gone."
The younger woman's home was also swept away in the same area, police said.
President Barack Obama ordered Federal Emergency Management Agency Administrator Craig Fugate to travel to Colorado Monday to "ensure the federal government is closely coordinating with the state and local response," the White House said Sunday.
Obama called Colorado Gov. John Hickenlooper to get an update on the situation and express his concern for residents affected by the historic flooding the National Weather Service called "biblical."
Rain and thick clouds grounded emergency aircraft that had been rescuing people, officials said.
"There's a heavy, heavy fog, and rain is coming down hard," the Post quoted Boulder County Emergency Management spokesman Andrew Barth as saying. "Standing water is rising because the ground is saturated."
Authorities across the flooded areas warned infrastructure, including new roads, would not be completed for many months.
More than 4,000 customers were without gas in Boulder County alone.
The flooding started a week ago as a slow-moving cold front stalled, clashing with warm humid monsoonal air from the south. The situation intensified Wednesday and Thursday. Boulder County was worst hit, with some 18 inches of rain recorded by Sunday.