Residents of more than 60 homes in the Field Points community have been told to evacuate, Chelan County Sheriff Mike Harum said.
The Deep Harbor already had burned through two campgrounds on the south shore of the lake, gobbling up a dock and picnic shelter.
The fire is part of a complex of fires in the Lake Chelan area that have burned about 20,000 acres.
The campgrounds at Graham Creek and Graham Harbor Creek, reachable only by boat, had previously been swathed in fire-protective wrap by firefighters, but the blaze burned the dock and picnic shelter at Graham Harbor Creek, spokeswoman Pam Sichting said.
No injuries were reported.
Deep Harbor, part of the Pot Peak-Sisi Ridge complex of wildfires northwest of the town of Chelan, was started by lightning on July 19, as was Sisi Ridge. The Pot Peak fire, about 15 miles northwest of here, was started by lightning June 26.
No towns were threatened by any of the fires. The Deep Harbor blaze did not affect boating on Lake Chelan, except for the noted campground closures, the Forest Service said.
As of Thursday evening, the Sisi Ridge fire, about 8 miles west of Stehekin, had burned across 345 acres and was 50 percent contained while Pot Peak had burned 15,500 acres and was 75 percent contained.
In all, 625 firefighters were working on the three fires, Sichting said.
In Western Washington, a smoky brush fire along Interstate 90 that threatened several homes temporarily closed the freeway's eastbound lanes near North Bend on Thursday evening.
The fire broke out around 4 p.m. and was contained about 5 p.m., said Josie Williams, a spokeswoman for Eastside Fire and Rescue.
Residents of about 14 houses in the nearby Cedar Village development of 132 homes were warned about the approaching flames, Williams said.
No homes were damaged and no injuries were reported.
The fire burned alongside the eastbound lanes between Exits 32 and 34 - roughly 30 miles east of Seattle.
The cause was under investigation.
