The South Fork of the river flows over granite bedrock, with portions of limestone and marble bedrock. The river has eroded the limestone and marble such that it travels underground in several locations. Tributaries on the South Fork include Rocky Creek, Turner Creek, Bixby Creek, Mill Creek, and Lachance Creek, many of them named for former homesteaders like Antare P. Lachance. Bixby Creek was the site of a landing built to transfer tanbark via cable to ships anchored offshore. The South Fork is unrestricted by any man-made dams, but an impassable waterfall about high upstream prevents steelhead from migrating further.