By Laila Kearney
SAN FRANCISCO, Aug 28 (Reuters) - A wildfire in Yosemite National Park that ranks as one of the largest California blazes on record was set to force the closure on Wednesday of a key route to the premiere outdoor destination and possibly block some visitors from reaching the park over the Labor Day weekend.
This comes a day after the so-called Rim Fire burned deeper into the park and reached the shores of a key reservoir that serves as the primary water supply for San Francisco some 200 miles (320 km) to the west.
The blaze has charred nearly 184,500 acres (about 75,000 hectares) - an area larger than the land mass of Chicago - since it erupted on Aug. 17.
It ranks as the biggest California wildfire since October 2007, and the sixth-largest in state history, according to the records of Cal Fire, a state government site.
The fire is burning mainly in the Stanislaus National Forest west of Yosemite, but it has scorched more than 40,000 acres of the park.
The Rim Fire last week forced the closure of a stretch of Highway 120 that leads to the west side of the 750,000-acre (300,000-hectare) Yosemite National Park and is the main entrance from the San Francisco Bay area.
On Wednesday at noon local time, the second of four access routes to the park was set to close when Tioga Road shuts down to allow fire crews to build containment lines along the road before the blaze approaches, said Yosemite spokesman Tom Medema.
"That will limit the access for visitors to and from the east side of the park, quite possibly over Labor Day weekend, which will have a significant economic impact on the area and an inconvenience for visitors," he said.
Some 4 million people visit Yosemite each year, most of them during the peak months of June through August.
Cooler temperatures, higher humidity and calmer winds expected overnight into Wednesday would likely help firefighters burn containment lines around the Rim Fire, said Alison Hesterly, spokeswoman for the California Department of Forestry and Fire Protection.
But later on Wednesday, temperatures were expected to be hot and dry, reaching a maximum of 94 Fahrenheit (34 Celsius) in the area with a minimum of 15 percent humidity, she said. "If we reach the maximum temperature and the minimum humidity, we're expecting continued erratic fire behavior," she added.
On Tuesday, a firefighting force of some 4,100 personnel, backed by teams of bulldozers and water-dropping helicopters, continued to make headway in their drive to encircle and suppress the flames.
Containment lines have been established around 20 percent of the fire's perimeter, nearly triple Sunday's figure, though the overall area of the blaze continued to grow as much of the firefighting effort focused on structure protection.
After advancing on the Hetch Hetchy Reservoir for several days, the flames reached the shores of the artificial lake on Tuesday, officials said. But the San Francisco Public Utilities Commission said in a statement there was "little risk for direct impacts" on the reservoir because of the rocky terrain and lack of brush surrounding it.
Officials said ash had drifted onto the surface of the Hetch Hetchy Reservoir, but testing of samples showed water quality remained healthy.
If the water should become fouled by too much ash and soot and require filtration, it can be diverted through a treatment plant near San Francisco before being delivered to customers, officials from the Public Utilities Commission said.
Firefighters hacking through dense, dry brush and trees to create clearings in the rugged terrain on Tuesday rushed to improve buffer zones around some 4,500 homes threatened by the blaze on its northwestern flank on Tuesday, said Cal Fire spokesman Daniel Berlant.
Most of those dwellings have been ordered evacuated or were under advisories urging residents to leave voluntarily or be ready to flee at a moment's notice. The fire has already destroyed dozens of homes and cabins, Berlant said, but no serious injuries have been reported.
The cause of the blaze remained under investigation. (Editing by Alex Dobuzinskis and Lisa Shumaker)