The South Fork Eel River is the largest tributary of the Eel River in north-central California in the United States. The river flows 105 miles (169 km) north from Laytonville to Dyerville/Founders' Grove where it joins the Eel River. The South Fork drains a long and narrow portion of the Coast Range of California in parts of Mendocino and Humboldt counties. U.S. Route 101 follows the river for much of its length.

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.