At the source of the Columbia River in Canal Flats, BC, there is a sign claiming that the nearby Kootenay River seeps underground and feeds water to the springs that are the source of the Columbia River. If that is the case, why is the Columbia River considered the main stem of the river?

At the source of the Columbia River in Canal Flats, BC, there is a sign claiming that the nearby Kootenay River seeps underground and feeds water to the springs that are the source of the Columbia River. If that is the case, why is the Columbia River considered the main stem of the river?