(CNN) -- A seventh person has been confirmed dead, and the number of presumed dead is now up to three after last week's flooding in Colorado, authorities reported Thursday.
The state Office of Emergency Management added a new confirmed fatality in Boulder County to the list Thursday afternoon, spokeswoman Micki Trost said. And in Larimer County, a 46-year-old man from the town of Drake whose home had been washed away was reclassified as presumed dead, said Nick Christensen, executive officer of the county Sheriff's Office.
Two other people in Larimer County -- both women, ages 60 and 80 -- were already presumed dead after their homes were swept away in the flash floods that ravaged Colorado's northern Front Range. Another 139 people remain unaccounted for in Larimer County, a figure reduced from nearly 200 on Wednesday, Christensen said.
Officials say most of those unaccounted for are expected to be alive and well, but have failed to check in with authorities to let them know.
The mountains north of Denver saw more than nine inches of rain on September 12 -- leading to flash floods that turned picturesque canyons into funnels that deluged towns downstream and left hundreds stranded in the mountains above. Nearly 1,200 people had been airlifted out of the mountains by Thursday, and Christensen said nearly 200 more have chosen to stay in their homes.
In addition to the deaths, nearly 18,000 homes have been damaged statewide. Gov. John Hickenlooper told reporters that about 200 miles of state highways washed out by the storms will need to be fixed, along with perhaps "several times more" miles of local roads.
Those roads are not only in hard-hit Boulder and Larimer counties but in farm communities northeast of Denver, where the runoff from the storms was carried downstream by the South Platte River. And he added, "It's clear we're in a race against the onset of winter."
"Critical roads needed to bring in crops that are about to go to harvest are completely broken apart," Hickenlooper said. "This has to be every bit as critical a priority as the roads connecting people with their communities and homes in the canyons. Irrigation, in large measure, has been washed away along the Platte."
The disaster may end up costing homeowners, businesses and local governments nearly $2 billion, according to the insurance consulting firm Eqecat -- and a high percentage of the losses will not be covered by insurance. Tom Larsen, senior vice president of of Eqecat, said that because very many of the homes damaged by the flooding are not in flood plains, the number of people with federal flood coverage is very low.
Colorado is also an oil-producing state, and the disaster has raised concerns that hydraulic fracturing, or "fracking," fluids used in oil and gas wells may have been swept up in the floodwaters and washed into the environment.
Colorado's Oil and Gas Association, the industry's trade association, said no fracking operations were underway when the floods hit -- meaning "no fracking fluids, no chemicals associated with fracking, nor equipment were on sites at the time of the flooding."
But state inspectors reported 10 oil spills as of Thursday, including two the Colorado Oil and Gas Conservation Commission called "notable" -- a 125-barrel (3,900-gallon) leak from a storage tank south of Milliken, near the South Platte; and a 323-barrel (13,500 gallon) leak from another storage tank near Firestone, on the St. Vrain River.
"In both cases, it appears the oil left the site in floodwaters," the commission reported. It said the spills were "promptly reported" by the tanks' owners, Anadarko Petroleum, and Anadarko said it was working with state and federal officials "to clean up the releases to the greatest extent possible."
And the state agency noted that oil is typically a small portion of the contaminants associated with a flood: "Those include very large volumes (millions of gallons) of raw, municipal sewage and other hazards associated with households, agriculture, business and industry."
CNN Correspondent Ana Cabrera and CNNMoney's Chris Isidore and Steve Hargreaves contributed to this report.