The COLUMBIA River is a North American river that runs through the Pacific Northwest region of the United States and Canada. It is the largest river in the Pacific Northwest, stretching 1, 243 miles from its source in the Rocky Mountains of British COLUMBIA to its mouth at the Pacific Ocean. The COLUMBIA River is the fourth-largest river in the United States by volume and the largest by discharge into the Pacific. It is also the second-largest river in the United States by drainage area, after the Mississippi River. The COLUMBIA River is an important source of hydroelectric power and irrigation water for the region.