Fire investigators have determined that lightning caused the 18,845-acre Thompson Fire in the Nyack drainage in the south-central area of Glacier National Park.

The Thompson Fire was first spotted Aug. 9. On Aug. 11 it exploded and grew almost 10,000 acres, prompting a variety of trail closures in the park.

Now that fire activity has slowed substantially, the Upper Two Medicine Lake, Dawson-Pitamakan, Cut Bank Creek and Triple Divide Pass trails have reopened to hikers.

The Cut Bank Pass trail remains closed as it drops down into the fire area.

Trails in the Nyack-Coal Creek drainage, the Ole Creek drainage and Park Creek drainages remain closed.

Firefighting operations are winding down on the Thompson Fire, although a Type 2 Incident Management Team is still managing the fire along with the Sheep and Granite fires to the south of the park. Park spokeswoman Michelle Fidler said it’s up to fire officials with the team to make the determination of when to transfer management of the fire.

“We’ll see what kind of weather comes in. If we get some precipitation, that will determine whether the team remains in place,” Fidler said.

The Thompson Fire has been relatively quiet for the past week, and on Friday the last firefighters completed burnout operations to reduce fuels around the northwest corner and were flown out. A small crew was flown back in Tuesday and Wednesday to remove hoses and pumps that had been left behind in case of increased fire activity.

Fire team spokesman Gregg Dinetto said operations are probably wrapped up on the fire, with park and fire officials beginning to discuss transferring management of the fire over to the park.

The team is concentrating on the Sheep Fire, which has experienced little activity this week following a cold front and return to more seasonal temperatures.

Essex residents were evacuated for four days last week after the 2,102-acre fire burned to within a mile of the small community. The evacuation was lifted Monday, although residents are still on a pre-evacuation notice in case fire activity increases again..

Fire crews have completed a fuel break from Dickey Creek to the BNSF railway trestle, with fire line built along Essex Creek. Structure protection is in place and crews are continuing to monitor the fire’s movement.

Although the Thompson Fire was determined to be lightning-caused, Fidler said Wednesday the Reynolds Creek Fire — which 4,850 acres near St. Mary Lake earlier this summer — is still under investigation after it was determined to be human-caused.

Reporter Samuel Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com.