The Columbia River is much more than the water flowing between its banks. Like any river, it is ecologically inseparable from its watershed. A watershed is the land area that delivers runoff, sediment, and dissolved substances to a river and its tributaries. In turn, the health of the watershed affects the temperature, flow rate, aquatic life, and other physical components of the river.
The Columbia’s watershed spans seven states and one Canadian province. The northernmost reach of the watershed is found in the high glaciers of the Canadian Rockies. From there, the main body (or stem) of the Columbia River runs over a thousand miles before reaching the Pacific. As the river runs south and west, it is fed by many smaller rivers before it is joined by the Snake River in Pasco, Washington. Near the confluence of the Snake River the Columbia River turns sharply west, forming a natural border between Washington and Oregon. On this leg of its journey, other rivers join the Columbia before it reaches the sea.
Of the rivers that feed into the Columbia, the Snake is the largest. In fact, the streams and small rivers feeding into the Snake represent 49% of the Columbia River Basins watershed below the Canadian border.
The watershed covers nearly 260,000 square miles, an area the size of France. Abundant precipitation from the hydrologic cycle, which is described in the section “What Makes The Columbia River Basin Unique,” provides the watershed with its seasonal supply of water.
Precipitation that does not infiltrate into the ground becomes runoff, and runoff from the watershed becomes the rivers, streams, wetlands, and lakes that we care about and enjoy. Some precipitation that seeps into the ground evaporates, but gravity pulls other water deeper into the earth. Sometimes this creates an underground river. This groundwater gathers in layers of underground rock and eventually becomes an aquifer, of which there are many in the Columbia River Basin.