Yosemite National Park will close another key route into thepark on Wednesday that could keep some visitors from reaching thepremier outdoor destination over the Labor Day weekend, accordingto a park spokesman.

The planned shutdown of Tioga Road comes a day after theso-called Rim Fire burned deeper into the park and reached theshores of a reservoir that serves as the primary water supply forSan Francisco some 200 miles to the west.

The blaze has charred nearly 187,500 acres - an area largerthan the land mass of Chicago – since it erupted on Aug. 17.Containment lines have been established around 23 percent of thefire's perimeter.

It ranks as the biggest California wildfire since October 2007and the sixth-largest in state history, according to the records ofCal Fire, a state government site.

The fire is burning mainly in the Stanislaus National Forestwest of Yosemite, but it has scorched more than 40,000 acres of thepark, and firefighters were making an extra push to stop it fromspreading inside Yosemite.

The fire last week forced the closure of a stretch of Highway120 that leads to the west side of the 750,000-acre park and is themain entrance from the San Francisco Bay area.

Tioga Road, the second of the four access routes into the park,was set to close at noon local time to allow fire crews to buildcontainment lines along the road before the blaze approaches, saidYosemite spokesman Tom Medema.

“That will limit the access for visitors to and from the eastside of the park, quite possibly over Labor Day weekend, which willhave a significant economic impact on the area and (be) aninconvenience for visitors,” he said.

Some 4 million people visit Yosemite each year, most of themduring the peak months of June through August.

Firefighters plan to burn containment lines from the HetchHetchy Reservoir in the remote northwestern section of the parksouth to Tioga Road to stop the fire from moving further east intothe park, Medema said.

By Wednesday afternoon, any remaining campers from the YosemiteCreek Campground and Tamarack Flat Campgrounds will be evacuated,he said. The park also closed the Crane Flat Campground, accordingto Medema.

HOT, DRY WEATHER EXPECTED

The blaze has been among the fastest-moving of dozens of largewildfires raging across the drought-parched U.S. West that havestrained national firefighting resources.

Cooler temperatures, higher humidity and calmer winds had beenexpected to help the firefighting effort Tuesday night, said AlisonHesterly, spokeswoman for the California Department of Forestry andFire Protection.

Later on Wednesday, temperatures were expected to be hot anddry, hitting a maximum of 94 Fahrenheit in the area with aminimum 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, backedby teams of bulldozers and water-dropping helicopters, continued tomake headway in their drive to encircle and suppress theflames.

After advancing on the Hetch Hetchy Reservoir for several days,flames advanced to the artificial lake on Tuesday, officials said.The San Francisco Public Utilities Commission said there was littlerisk to the reservoir because of the rocky terrain and lack ofbrush surrounding it.

Officials said ash had drifted onto the surface of thereservoir, but testing of samples showed water quality remainedhealthy.

If the water should become fouled by too much ash and soot andrequire filtration, it can be diverted through a treatment plantnear San Francisco before being delivered to customers, officialsfrom the commission said.

Firefighters hacking through dense, dry brush and trees tocreate clearings in the rugged terrain on Tuesday rushed to improvebuffer zones around some 4,500 homes outside Yosemite that werethreatened by the blaze on its northwestern flank on Tuesday, saidCal Fire spokesman Daniel Berlant.

Most of those dwellings have been ordered evacuated or wereunder advisories urging residents to leave voluntarily or be readyto flee at a moment's notice. The fire has already destroyed dozensof homes and cabins, Berlant said, but no serious injuries havebeen reported.

The cause of the blaze remained under investigation.