One of the largest undammed river systems in California, the South Fork drains a rugged, remote watershed of 980 square miles (2,500 km2).

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.

[3] 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.

[1] 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.

Below here the river passes Forest Glen, receives Rattlesnake Creek from the east, and crosses underneath California State Route 36.

[1][4][5] 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.

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.

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.

While along the length of the South Fork itself there is little human development, it receives agricultural pollutants from Hayfork Creek, whose valley contains over 52,000 acres (210 km2) of ranchlands and farmlands.

Clearing hillsides has accelerated erosion, clouding the water and causing difficulties for steelhead trout and chinook salmon, which once spawned in the river in prodigious numbers.

One particular trail crossed through the Yolla Bolly Mountains, then wound down to Hayfork Creek and west to the South Fork Trinity.

It was not long before debris and silt began to cloud the creeks feeding the South Fork, many of which were still located on United States Forest Service land.

In the Christmas flood of 1964, heavy rains washed enormous amounts of silt and debris from clear-cut lands into the river, and killing entire fish and amphibian populations.

[6] Remaining old-growth forests in the South Fork watershed provide vital habitat for several threatened and near-threatened species, including the northern spotted owl.

Other species in the area include a variety of ducks, river otters and minks, coyotes, rabbits, Gray and Ground squirrels, and Chipmunks.