The South Fork Trinity River is the main tributary of the Trinity River, in the northern part of the U.S. state of California.
It is part of the Klamath River drainage basin. It flows generally northwest from its source in the Klamath Mountains, 92 miles (148 km) through Humboldt and Trinity Counties, to join the Trinity near Salyer.
The main tributaries are Hayfork Creek and the East Fork South Fork Trinity River.
The river has no major dams or diversions, and is designated Wild and Scenic for its entire length.
One of the largest undammed river systems in California, the South Fork drains a rugged, remote watershed of 980 square miles (2,500 km2). The large areas of intact habitat are important for several endangered species and rare plants. Historically, the South Fork watershed was known for its prodigious anadromous fish population and dense old-growth forests.
