premise: The Columbia River undergoes drainage.
hypothesis: Columbia River . The river is 1243 mi long , and its largest tributary is the Snake River . Its drainage basin is roughly the size of France and extends into seven US states and a Canadian province . Overland explorers entered the Willamette Valley through the scenic but treacherous Columbia River Gorge , and pioneers began to settle the valley in increasing numbers . Since the early 20th century , dams have been built across the river for power generation , navigation , irrigation , and flood control .