Trinity River (California)

Happy Camp, California

June 10, 2021 Not to be confused with Trinity River (Texas)

The Trinity River (originally called the Hoopa or Hupa by the Yurok, and hun' by the Natinixwe/Hupa people) is a major river in northwestern California in the United States, and is the principal tributary of the Klamath River. The Trinity flows for 165 miles through the Klamath Mountains and Coast Ranges, with a watershed area of nearly 3,000 square miles in Trinity and Humboldt counties. Designated a National Wild and Scenic River, along most of its course the Trinity flows swiftly through tight canyons and mountain meadows. The river is known for its once prolific runs of anadromous fish, notably Chinook salmon and steelhead, which sustained Native American tribes for thousands of years. Due to its remoteness, the Trinity did not feature prominently in the early European colonization of California, but the gold rush in the mid-1800s brought thousands of gold seekers to the area. The river was named by Major Pierson B. Reading who, upon reaching the river in 1848, mistakenly believed it to flow into the Pacific Ocean at Trinidad Bay. During and after the gold rush, the influx of settlers and miners into the Trinity River country led to conflict with indigenous tribes, many of which saw severe depopulation due to fighting and foreign diseases. In the following decades logging and ranching, combined with mining runoff, significantly changed the river's ecology and led to the decline of its fish populations.

Today, the Trinity River is an important water source for irrigation and hydroelectricity generation, as well as a major center of recreational activities such as gold panning, fishing and whitewater rafting. Since 1964 the Trinity River has been dammed to create Trinity Lake, the third largest man-made lake in the state. As much as 90 percent of the upper Trinity River watershed was diverted for agriculture in the Central Valley. In 1991 environmental regulations were enacted, requiring a greater release of water to the Trinity River in order to protect fish. However, the use of Trinity River water remains a contentious issue, especially in years of drought. Geology

The lands that make up the Trinity River basin today began to take shape over 200 million years ago by the collision of several exotic terranes – or crustal fragments of the Pacific Plate – with the North American Plate, causing uplift of the sea floor under what is now northwestern California. During the Jurassic and Cretaceous periods, tectonic movement along the plate boundary produced a range of mountains much higher than those found in the area today. Over millions of years these mountains were eroded, then reformed again as the next oceanic terrane collided with the continental crust. This repeating cycle of erosion and progeny created the complex "jumble of different rock types" that characterizes the region today. Rocks commonly found in the Trinity River area include gabbro, chert, granite, diorite, limestone, sandstone, serpentine, schist and marble. Gold-bearing quartz veins are widespread in local metamorphic rockformations; the richness of the area made it among the focal points of the California Gold Rush.

The Klamath Mountains, which make up the eastern part of the watershed are quite young in geologic terms, no more than 2 or 3 million years old. The present shape of the mountains was highly influenced by underground volcanic activity, which created batholiths, domes of igneous rockformed by cooled magma. They raised the elevation of the terrain above and created the widespread granite and diorite formations found in the area today. The higher mountains, including the Trinity Alps – the highest range in northwest California – were also sculpted by glaciation during successive Ice Ages, the last of which ended roughly 10,000 years ago. Glacial erosion produced numerous granite outcroppings, tarns, cirques and knife-edged ridges. Remnants of these glaciers, or "glacierets", are still extant in the higher valleys.

The Coast Ranges pass through the western part of the Trinity River Basin and consist of even younger rock formations, chiefly the Franciscan Assemblage. The Franciscan formation consists of more unstable sedimentary and igneous rocks and soils that are highly breakable and prone to erosion. They formed even more recently in geologic history than the Klamath Mountains, primarily due to uplift along the Cascadia subduction zone. The Natural Resources Conservation Service refers to the Franciscan assemblage as a "nightmare of rocks" due to its complex and fragmented layers. The most common type of rock is greywacke, followed by other types of sandstones and shales. Landslides and mass wasting are common in this region due to erosion as well as earthquakes.

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