The fire started in rough, roadless country along the Wild and Scenic section of the Middle Fork. The Bear Fire was also spotted around 9:00 AM on August 17, in the Middle Fork canyon west of the Claremont Fire near the Pacific Crest Trail.
On the morning of September 4 it had grown to 24,330 acres (9,850 ha), and containment had dropped to 49 percent. On the morning of August 31, the North Complex Fire was at 22,780 acres (9,220 ha), and 59 percent contained. On September 5, with fire activity much lower on the eastern sides, evacuation orders were lifted for Spring Garden, Greenhorn, Sloat and Cromberg. On September 1 mandatory evacuations were lifted for Quincy-La Porte Road. On August 30 crews conducted backfire operations on the eastern side of the fire and prevented it from spreading towards Spring Garden and Cromberg. Mandatory evacuations were lifted in East Quincy, but with continued strong west winds, Greenhorn and Sloat to the east were evacuated on August 28. On August 27, the Claremont Fire was at 20,697 acres (8,376 ha) and 47 percent contained.
The fire threatened State Route 70 and the Union Pacific Railroad tracks, shutting both down intermittently for several days. On August 23, aided by high winds, the fire began advancing rapidly east forcing Spring Garden and Greenhorn to be evacuated. On August 22 the fire jumped California State Route 70 at the Massack Rest Area southeast of Quincy. On August 20 an evacuation advisory was issued for East Quincy which was quickly changed to a mandatory evacuation for East Quincy and residents along Quincy-La Porte Road. The fire originated along Claremont Creek, a tributary of the Middle Fork just south of Quincy, and quickly jumped the ridge northwards towards American Valley. The Claremont Fire was spotted around 9:00 AM PDT on August 17. The Sheep Fire burned about 29,570 acres (11,970 ha), before it was fully contained on September 9. The Sheep Fire near Susanville was originally included as part of the North Complex, but on September 5, it was assigned to a separate incident. By August 22, 16 of the original 21 fires had been contained. The storms came from a large plume of moisture from Tropical Storm Fausto that were carried into California, which started hundreds of wildfires simultaneously across Northern and Central California. On August 17, 2020, dry thunderstorms sparked 21 wildfires in the Plumas National Forest and Lassen National Forest, the largest of which were the Claremont and Bear Fires along the canyon of the Middle Fork Feather River. The North Complex Fire was the sixth-largest in California's modern history, and the deadliest fire in the 2020 California wildfire season. Forest Service in conjunction with Cal Fire, with the primary incident base in Quincy. The complex burned an estimated 318,935 acres (129,068 ha), and was 100% contained on December 3. Among the 16 fatalities was a 16-year-old boy. The fire killed 16 people and injured more than 100. The fire threatened the city of Oroville, before its westward spread was stopped. On September 8, 2020, the towns of Berry Creek and Feather Falls were immediately evacuated at 3:15 PM PDT with no prior warning, By September 9, 2020, the towns of Berry Creek and Feather Falls had been leveled, with few homes left standing. On September 8, strong winds caused the Bear/Claremont Fire to explode in size, rapidly spreading to the southwest. 21 fires were started by lightning on Augby September 5, all the individual fires had been put out with the exception of the Claremont and Bear Fires, which merged on that date, and the Sheep Fire, which was then designated a separate incident. The North Complex Fire was a massive wildfire complex that burned in the Plumas National Forest in Northern California in the counties of Plumas and Butte.