Bull Creek is the largest Eel River tributary drainage basin preserved within Humboldt Redwoods State Park. The basin contains the world's largest remaining contiguous old-growth forest of coast redwoods. Bull Creek flows in a clockwise semi-circle around 3,373-foot (1,028-meter) Grasshopper Mountain to enter the South Fork Eel River approximately 1.5 miles (2.4 km) upstream of the South Fork confluence with the Eel River.

The Eel River is a major river, about 196 miles (315 km) long, in northwestern California. The river and its tributaries form the third-largest watershed entirely in California, draining a rugged area of 3,684 square miles (9,540 km2) in five counties. The river flows generally northward through the Coast Ranges west of the Sacramento Valley, emptying into the Pacific Ocean about 10 miles (16 km) downstream from Fortuna and just south of Humboldt Bay. The river provides groundwater recharge, recreation, and industrial, agricultural and municipal water supply.