Marine engineers from Camp Pendleton were detonating explosives in the Cleveland National Forest around the time of the Jim fire's ignition Wednesday morning, authorities have confirmed.
The fire, which has been burning for two days inside the forest near the Riverside-Orange County line, was 50% contained and holding at 553 acres Friday morning.
The official cause of the fire remains under investigation. Cleveland National Forest Service spokesman Nathan Judy acknowledged that crews were in the forest around the time of ignition.
"We did have crews working in the area — there is a project going on back in there — but the cause is still under investigation," he said.
The blasts were part of a dam removal operation conducted by the U.S. Forest Service, the Marine Corps and other federal agencies aimed at restoring wildlife habitats in the forest, as first reported by the Desert Sun.
In 2017, the project drew fervent opposition from local residents and conservation groups who feared demolition would clog creeks with debris, kill aquatic wildlife and wipe out popular swimming holes.
The project targets 81 dams for removal, according to a Forest Service environmental assessment. A spokesperson for Camp Pendleton did not immediately respond to a request for comment.
The Jim fire sparked around 11:20 a.m. in the Holy Jim Canyon area of the forest and quickly spread uphill, mushrooming from 10 to 400 acres in its first three hours.
No homes were threatened by the fire and no evacuations were ordered, officials said.
Like most of the state, Southern California has experienced an exceptionally dry start to the year. The months of January and February were the driest ever recorded.
The National Weather Service said their closest monitoring station to the fire, which is at a lower elevation than where the blaze is burning, recorded a scant 0.21 inches of precipitation in January and February, compared to the normal amount of about 6 inches.
Judy said the grasses were moist on Wednesday, but larger fuels, such as trees, were drier. The fire danger Wednesday was classified as "moderate," he said.
The conditions helped fuel a second wildfire in the forest that ignited Thursday but was quickly controlled.
Dubbed the San Juan fire, it sparked around 12:15 p.m. off State Route 74 near Sievers Canyon and grew to about 9 acres.
As of Friday morning, it was 90% contained. The cause of that fire also remains under investigation.
Judy said about 250 personnel were still assigned to the Jim fire on Friday, with hand crews and bulldozers laying containment lines on the ground and helicopters dropping water from above.
The outlook for the day was promising as a cool weather system moves into the area along with some clouds and rain.
"We're going to have air tankers on standby if they need those, and helicopters as well to be dusting hotspots," he said. "But with precipitation expected, that should help out any kind of smoldering areas over the fire."
