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Trinity River Watershed Management Area


  • Upper Trinity River Watershed Analysis
  • The Trinity River basin drains an area of approximately 2,900 square miles of mountainous terrain. The Trinity River is the largest tributary to the Klamath River; from its headwaters in the Klamath and Coast ranges, the river flows 172 miles south and west through Trinity County, then north through Humboldt County and the Hoopa Valley and Yurok Indian reservations to its confluence with the Klamath River.

Much of the watershed management area (WMA) is prone to seismically induced landslides, especially during winter months when soils are saturated. Additionally, inner valley gorges are considered highly unstable. Ground water resources are relatively plentiful throughout the WMA, but are not well defined.

Annual precipitation averages 57 inches per year with a low of 37 inches in Weaverville and Hayfork, and with higher rainfalls of 75 inches in Trinity Center and 85 inches in the Hoopa Mountains. There are occasional summer thunderstorms that produce extensive runoff and may start wildfires.

The Trinity River watershed is primarily rural with human populations centered near Trinity Center, Weaverville, Lewiston, Hayfork and Hyampom. Timber harvest has traditionally been a large factor in the economy on both federal and private land. The US Forest Service (USFS) and the Bureau of Land Management (BLM) manage approximately 80 percent of the land in the Trinity WMA; of the remaining 20 percent, about half are industrial timberlands.

In the early 1950s two major water-development features were installed above river-mile 112 and the community of Lewiston. This "Trinity River Diversion (TRD)" consists of Lewiston Dam and its reservoir and related facilities, and Trinity Dam and its reservoir (known as Trinity Lake).

The TRD project diverts a majority of the upper-basin’s water yield at Lewiston for power generation and to support the US Bureau of Reclamation’s (USBR) Central Valley Project (CVP). The hydrologic changes produced by the TRD project have altered stream-channel conditions and instream habitat for many miles below Lewiston.

Trinity River downstream of the TRD provides habitat not only for anadromous salmonids and other native species, but also the non-native brown trout (Salmo trutta).

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