The Columbia River, which forms much of Oregon's northern border, also played a major role in the region's geological evolution, as well as its economic and cultural development. The Columbia is one of North America's largest rivers, and one of two rivers to cut through the Cascades (the Klamath River in southern Oregon is the other). About 15,000 years ago, the Columbia repeatedly flooded much of Oregon during the Missoula Floods; the modern fertility of the Willamette Valley is largely the result. Plentiful salmon made parts of the river, such as Celilo Falls, hubs of economic activity for thousands of years.