Austen Thibault, The Union Democrat

The Power Fire, burning in a remote stretch of the Middle Fork Stanislaus River canyon just east of Beardsley Reservoir, has grown to more than 800 acres, according to U.S. Forest Service officials.

Firefighters had about 25 percent of the fire surrounded, or "contained," this morning. Its rate of spread was described as "moderate" with a high growth potential, according to Bob Poole, Forest Service fire information officer.

It's moving northeast, along the east bank of the Middle Fork of the Stanislaus River, and mostly parallel to Highway 108.

No structures are threatened.

The fire was reported at 5:30 a.m. Monday as a one-to-four acre fire near the Donnell powerhouse, directly upstream from Beardsley Reservoir.

The cause has not been determined.

Tuolumne and Calaveras County public officials have received numerous complaints about heavy smoke.

Readings put the air quality in the "moderate" range, according to Vicki Helmar, Tuolumne County air pollution control officer.

The elderly or people with health problems should stay indoors and avoid exerting themselves, as these conditions are expected for the next few days, she said at the Tuolumne County Board of Supervisors meeting Tuesday.

"Keep your windows closed at night, that is when the smoke is going to come down," she advised to residents downhill of the fire.

A change in weather may come at the end of the week and kick up more smoke during the day with wind from the north, she said.

So far, winds have been normal for the area, said Jerry Snyder, Stanislaus National Forest spokesman.

But, "traversing that very steep ground is pretty hazardous for our firefighters," he said.

At least 365 personnel - most from the Forest Service - have responded to the blaze which is in extremely difficult terrain, according to the Stanislaus National Forest.

A few of them have suffered minor injuries, said Snyder, including a dislocated shoulder.

Firefighters have reported more falling trees than they're used to, Snyder said, including one very close call.

A trunk about 40 inches around fell on the cab of a fire engine, as all of the crew was out laying hose from the engine.

Conditions are very dry with lots of underbrush, he said, with canyon winds that blow both up and downhill at different times of day.

Winds go up-canyon during the day and down-canyon at night. The fire is spreading across a ridge that reaches about 6,000 feet elevation, he said.

A fair distance away are cabins and tract homes that the Forest Service is concerned about, but overall he thinks residents are "really pretty safe there in the Pinecrest area."

The greatest impact will be smoke, he said.

Competing needs for firefighting resources have also been an issue, with other fires in the state, but resources are returning from the 20,000-acre Aspen Fire near Clovis, now 75 percent contained.

Union Democrat reporter Chris Caskey contributed to this report.