A tributary is a river or stream that feeds into a larger body of water. The Snake River, originating in Yellowstone Park and joining the Columbia near Hanford, is the Columbia’s largest tributary. As water flows downstream, organic debris including decomposing organisms and leaves collect and provide nutrients to plants, fish, and wildlife.

The Columbia River’s tributaries originate in mountainous areas characterized by subalpine forest. Here, the gradient is steep and stream velocity is high. As the Columbia’s tributaries flow into the interior of the basin, pine and fir trees dominate the forest. In contrast to the higher elevations, the climate is drier, the gradient gentler, and the stream flow reduced. At the rivers mouth, forests include cedar and western hemlock. These trees thrive on heavy precipitation and a moist environment.

The Columbia’s tributaries are also strongly affected by seasonal runoff. Precipitation, primarily from winter snowfall, dramatically increases runoff from snowmelt during spring and early summer. This can cause flooding when unseasonally warm rains or winds occur.

Plants, fish, and wildlife thrive around tributaries. Many large mammals and semi-aquatic species like the playful river otter make tributaries a part of the territory they call home. Song sparrows and other small birds and mammals also live by these waters, particularly where insects are plentiful and forest cover is nearby. Fish such as the bull trout and cutthroat trout spawn in tributary headwaters, and mountain whitefish thrive in the cold, fast-moving areas of large streams.

The seven species of Pacific salmon (chinook, sockeye, coho, chum, pink, steelhead, and cutthroat trout) originate in the tributaries of the Columbia River Basin. When salmon fry emerge from their eggs, the specific tributary in which they are born is imprinted into their genetic code. They begin their descent downstream toward the ocean, allowing the current to sweep them along. When they return to the river as adults to perpetuate their life cycle, they miraculously return to the same tributary where they were hatched. For this reason, the health of tributaries is important in ensuring the genetic diversity of salmon and other migratory fish.