Also it's cheaper to ship grain from the upper Missouri region to the Columbia Plateau and then down the Snake and Columbia on barges where it then gets loaded onto freighters in Portland, Vancouver and Longview and shipped to Asia and South America. By volume the Columbia is the second largest watershed in the US. Obviously all of the Columbia Plateau grain gets shipped down the Columbia as well, which is a lot, since it's a major grain producing region.

The downside is crossing the Columbia Bar which is also known as the graveyard of the Pacific, but there are highly paid Columbia River pilots for that.