The South Fork Eel River begins near Iron Mountain in western
Mendocino County, at an elevation of . It is not long after its headwaters that Branscomb Road drops into the South Fork Eel's canyon from the north, paralleling the river. Almost past this point, it receives its first significant
tributary, Section Four Creek, on the left bank. Although it is not a large creek, only about long, it denotes the start of the
National Wild and Scenic River section of the South Fork. The river meanders swiftly west and turns west-northwest, passing
Branscomb, California. Near the city, it receives Rock Creek on the right and Redwood Creek on the left. These two stream names are a common occurrence throughout the South Fork's watershed. Afterwards, it receives its first significant tributary, Tenmile Creek, on the right bank. Tenmile Creek begins in another section of the Coast Range, separated from the South Fork Eel River by two sub-ranges. The creek begins in the easternmost of these two sub-ranges. It then flows west, cutting a
water gap through the western subrange, and spills into the South Fork. The creek is about long, despite the name. After the confluence with Tenmile Creek, the South Fork flows generally north, turning west where it receives another major tributary, Rattlesnake Creek, also on the right. This point is significant because it is where it begins to parallel
U.S. Highway 101 and
California State Route 271 (Redwood Highway and Old Redwood Highway). Both roads come in from the east and at this point are on the river's right bank. The river turns northwest, receiving Big Dann Creek and another large tributary,
Cedar Creek, on the right bank. Cedar Creek flows west and turns south-southwest, flowing also in a steep, undeveloped gorge. Shortly past Cedar Creek, the South Fork Eel meets another large tributary, Hollow Tree Creek. Hollow Tree Creek flows east, turns north, then turns east again to meet the South Fork, fed by several smaller creeks. The South Fork then turns west again, flowing through the
Standish Hickey State Recreational Area. It then meanders north into
Richardson Grove State Park on an increasingly wide valley floor, receiving Red Mountain Creek on the right bank. The river then reaches Benbow Lake, which is inside the
Benbow State Recreation Area and next to the town of
Benbow, California. Benbow Lake was a seasonal
reservoir, formed by a dam at its western end. The dam was only raised in the summer, and only when water flow is sufficient for impoundment. Since about 2009, the lake no longer gets built, which is good for the ecology of the river.{{cite web in the
Humboldt Redwoods State Park The South Fork then flows north to a point where it turns sharply around a ridge and flows due south, then turns back north again. At this point, it is paralleled by Highway 101 on the left bank and by
California State Route 254 (Avenue of the Giants) on the right bank. U.S. 101 then crosses the South Fork, paralleling CA-254. The village of
Myers Flat is located on a low slice of terrain north of the river. The South Fork, nearing its mouth, passes
Burlington on the right bank, and
Weott, California, also on the right bank. Several hundred yards upstream from its confluence, it receives its last major tributary, Bull Creek, on the left bank. Bull Creek, whose watershed is entirely contained inside
Humboldt Redwoods State Park,{{cite web url=http://www.mountainzone.com/mountains/detail.asp?fid=619645 and receives its last named tributary, Cabin Creek, on the left bank. Meandering through a downcut channel between U.S. 101 and SR 254 (right bank) and
California State Route 211, it then crosses under the former two highways, spreads wider, and flows into the Eel River. ==Watershed==