Firefighters and Portland officials are crossing their fingers that the

near Mount Hood will steer clear of the

, the main source of drinking water for 900,000 people in Portland and many of its suburbs.

The 4,500-acre fire south of Hood River is three miles east of the watershed boundary and only 10 percent contained. The fire, which began after a lighting strike on Aug. 27, was relatively calm in today's light winds, but could spread west given rising temperatures, dry air and east winds with gusts up to 30 mph expected to kick up early Wednesday morning.

More than 730 firefighters are attacking the blaze. The fire would have to make a significant run and burn uncharacteristically downhill to reach the watershed, firefighters say.

They're hoping the winds will blow the fire to the southwest, away from the watershed, but add that wind directions are tough to predict in the rugged terrain between the mountain and the Bull Run.

"We just have to see what the wind deals us," said Peter Frenzen, spokesman for a national fire team assigned to the incident. "It all depends on how long and how strong and how sustained it is."

A fire wouldn't immediately affect water quality,

Administrator David Shaff said. But a large burn in the 68,000-acre watershed could send debris, dirt and ash into the water once rains begin, potentially causing a shutdown of the unfiltered system. The city has

, but their capacity is far lower than the two Bull Run reservoirs.

Working in the city's favor: The fire is closest to the eastern edge of the Bull Run management unit, miles away from the two reservoirs. Shaff said the

, which owns the land, is well aware of the threat.

"Next to the safety of their staff, (the Bull Run) is the highest priority of the Forest Service as far as fighting this fire," Shaff said.

Frenzen said the biggest threat is that wind-blown embers could start spot fires beyond fire lines being cut between the blaze, the watershed and high-voltage power lines that run inbetween. Crews are prepared to fight spot fires, though the rough, steep terrain makes firefighting more difficult. More crews are on the way, he said.

The watershed is thickly forested. After years of controversy, 1996 congressional legislation barred Bull Run logging in an effort to reduce erosion and the resulting sediment in the water supply.

Access to the Bull Run is tightly restricted. The last fire in the watershed, a small lightning-caused fire, ignited in August 2008.

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