South Fork Trinity River

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. During the mid-20th century, the river channel was heavily damaged by major flooding, which was exacerbated by erosion caused by mining, logging and ranching. Decades later the South Fork is still considered in the process of recovery.

The South Fork Trinity River begins as a small spring on the west slope of Chicago Rock near North Yolla Bolly Mountain, 4,460 feet (1,360 m) above sea level. From there, it flows briefly west and turns to the north, receiving numerous small tributaries which drain a series of steep, forested valleys in the headwaters. At about 4 miles (6.4 km) from its source, the river is crossed by the Humboldt Trail, then it receives north-flowing Shell Mountain Creek,from the west. It also receives Pal Creek, Raspberry Gulch, and Mule Creek. Shortly downstream, the 10-mile (16 km) long East Fork South Fork Trinity River joins from the east. Shortly afterwards, it receives Happy Camp Creek from the west and Smoky Creek from the east.

Below here the river passes Forest Glen, receives Rattlesnake Creek from the east, and crosses underneath California State Route 36. Here the river runs roughly parallel to the Mad River, separated by a 2,000-foot (610 m) divide to the west. Below State Route 36, it receives Butter Creek and Deep Gulch, both from the east.

The river then enters the wide Hyampom Valley, where it passes the town of Hyampom and receives its biggest tributary, Hayfork Creek, from the east. It then passes the Hyampom Airport and receives Pelletreau Creek, Kerlin Creek, and Mill Creek from the west. Within the valley the river briefly exhibits braided characteristics, with a wide floodplain. At the north end of the valley the river enters another canyon, receiving Mingo Creek from the west, then veers sharply eastward and then turns sharply north again. The river receives Madden Creek from the west and crosses underneath California State Route 299. Directly below the bridge, the South Fork flows north into the Trinity River.

Over hundreds of millions of years, the westward movement of North America caused it to accrete many terranes from the Pacific Ocean region along its west coast. Four major terranes have so far collided with the northwest coast of California—the oldest dating to pre-Jurassic times—crumpling the crust upwards into the 10,000-foot (3,000 m)-high massif of the Klamath Mountains. Most of the Klamath Mountains consist of granite and batholiths underlie most of the major peaks. The second most recent of the terranes—dating to the Cretaceous—which is composed almost entirely of granite, brought with it a strip of mica that roughly aligns with the present course of the South Fork Trinity River. The mica caused the granite to become weaker than the surrounding rock, so this area was subjected to greater erosion that created the valley of the South Fork, the lower Trinity River, and the lower Klamath River. This is also why the Klamath and Trinity rivers have this sharp northwest bend on their generally southwest courses, and the South Fork Trinity River valley is the southernmost extension of the roughly 200-mile (320 km)-long gorge formed by this abundance of mica.

The vast majority of the South Fork watershed is mountains, with the only level land found in the Hyampom Valley at the confluence of the river and Hayfork Creek, along the Hayfork valley, and along narrow river terraces. There are large parts of the watershed where the ground is composed of stable bedrock, while large portions of hillsides are composed of loose soil and rock. Historically, riverbeds in the watershed were narrow and rocky, but due to vast amounts of silt washed down by poor logging practices, streambeds have become wide, braided, elevated and shallow.

The river and its tributaries drain 980 square miles (2,500 km²) in Trinity County in the south and Humboldt County and Trinity County in the north and comprising 34 percent of the 2,853-square-mile (7,390 km²) Trinity River watershed. The Hayfork Creek sub-watershed contains 379 square miles (980 km²), or 38 percent of the entire South Fork Trinity watershed. Most of the watershed lies on public lands (79 percent), while for Hayfork Creek, 78 percent of its watershed lies on public lands. With the Yolla Bolly Mountains in the south and the Klamath Mountains in the north, the topography of the South Fork Trinity's watershed is dissected by deep gorges and valleys separated by narrow ridges. The South Fork is the longest undammed National Wild and Scenic River in California. (The Eel River, also a Wild and Scenic River, is over twice as long, but is dammed near its headwaters. The Smith River drains a greater area but is much shorter.)