Jul. 13—Firefighters struggled to gain ground against the Beckwourth Complex — the state's largest wildfire — which ravaged more than 90,000 acres of Plumas County as of Monday evening, displacing thousands.

Officials began allowing some of the 3,000 evacuated residents to return home Monday as firefighters continued to make progress battling the eight-day old blaze, U.S. Forest Service officials said at a briefing. On Monday, ground crews carved containment lines into the rugged terrain and aircraft crews dropped flame retardant to keep the wildfire contained at 26%.

The wildfire is a merger of two fires, the Dotta Fire — which sparked on June 30 — and the Sugar Fire, which sparked on July 2. Both fires were started by lightning near the Plumas National Forest.

Despite continuing to stockpile equipment on the western side of the fire complex, Operations Section Chief Jake Cagle said officials were "confident" that firefighting efforts would overcome bone-dry conditions and a string of 100-degree days that prompted meteorologists to issue excessive heat warnings for the area.

"We are making some really good ground," Cagle said Monday evening.

The combination of steep terrain and a strong wind flow into the base of fire continued to challenge firefighters' efforts at containment, incident meteorologist Dan Harty said Monday evening.

The erratic winds were causing the fire to "create its own weather," Harty said, but calmer conditions were expected to prevail overnight, allowing firefighters to make ground.

Firefighters braved a scorching hot afternoon with temperatures in the triple digits for the eighth day, about 15 degrees above normal in the region around the fire. Relative humidity had also dropped, fueling added concerns about the potential for rapacious growth in the parched forest.

Vegetation in the area where the fire is raging is desiccated at levels not usually observed until much later in the year, fire analyst Diondray Wiley said.

Elsewhere, the River Fire in Madera and Mariposa counties spread across 9,000 acres into the Sierra Nevada Foothills as of Monday evening. The wildfire has forced hundreds to evacuate and was 10% contained, according to officials.

Chronicle staff writer Omar Rashad contributed to this report.

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