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 Branch South Fork Trinity River.