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 South Fork is designated as a National Wild and Scenic River from the confluence of Section Four Creek to the mouth.