The Columbia receives more than 60 significant tributaries. The four largest that empty directly into the Columbia (measured either by discharge or by size of watershed) are the Snake River (mostly in Idaho), the Willamette River (in northwest Oregon), the Kootenay River (mostly in British Columbia), and the Pend Oreille River (mostly in northern Washington and Idaho, also known as the lower part of the Clark Fork). Each of these four averages more than 20,000 cubic feet per second (570 m3/s) and drains an area of more than 20,000 square miles (52,000 km2).

The Snake is by far the largest tributary. Its watershed of 108,000 square miles (280,000 km2) is larger than the state of Idaho. Its discharge is roughly a third of the Columbia's at the rivers' confluence.

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