The Kootenay (Kootenai in the U.S. and historically called the Flatbow) is a major river in southeastern British Columbia, Canada and the northern part of the U.S. states of Montana and Idaho. It is one of the uppermost major tributaries of the Columbia River, which is the largest North American river that empties into the Pacific Ocean. The Kootenay River runs 781 kilometres (485 mi) from its origins in the Kootenay Ranges of the Canadian Rockies, exiting British Columbia's East Kootenay region via northwestern Montana and the northernmost Idaho Panhandle to return to Canada in the West Kootenay region, where it ends at the city of Castlegar, British Columbia.

Born in glaciers and flowing through a rugged landscape of mountains and valleys, this river drains an isolated and sparsely populated region of the Pacific Northwest. Between the highest headwaters and the Columbia River, the Kootenay’s elevation dips by over two kilometers. Although comparable by length, watershed and discharge to the Columbia above where the two rivers meet, the Kootenay is of a very different character. Its dramatic drop, caused by a steep gradient, results in the formation of many rapids.