The South Fork Eel River flows from its headwaters at Cahto Peak near Laytonville in Mendocino County to its confluence with the mainstem Eel River near Weott in Humboldt County. The river flows mainly from south to north and is approximately 105 miles long. At 689 square miles, the South Fork Eel Basin is the second largest basin in the Eel River system. Elevations within the basin range from 100 feet at the confluence with the Eel River to 4,491 feet at Iron Peak.
Rugged terrain throughout the basin is underlain by soft Franciscan geology, which combines with high winter precipitation to result in high instream sediment loads.
The eastern side of the basin is composed of oak woodlands and grasslands with some redwood in lower drainage while the western part is dominated by redwood and Douglas fir forest. The predominant land uses throughout the basin are timber harvest, livestock grazing, and dispersed rural development. Approximately 80% of the basin is privately owned. Highway 101 runs along much of the South Fork Eel River and provides a major thoroughfare for travel.
The South Fork Eel River from its confluence with the mainstem to the Section Four Creek confluence is designated as a Wild and Scenic River. The section of the river in the BLM Angelo Coast Range Preserve is designated as "wild" and the remainder is designated as "recreational." The South Fork is also is listed on the National Rivers Inventory, a list of potential wild, scenic, and recreational river areas within the United States. The river is listed for two outstandingly remarkable values: scenery and fish (NPS 2004).
The basin supports runs of coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss), and Chinook salmon (O. tshawytscha). It is thought that the South Fork was once the most productive major tributary of the Eel River for anadromous salmonids and once supported half the total coho salmon run for California (Fuller 1996). However, fish populations declined in the 20th century, especially after the huge 1955 and 1964 floods (HCRCD 2002). In recent years, efforts have been underway to recover salmonid stocks of the South Fork Basin.