Since colonialists first entered the southwestern United States, they’ve needed a river to secure their futures. Empires are built on rivers and the United States, during westward expansion, is no different. The Colorado River is a product of colonial imaginary, to take the various waters flowing from mountains and through canyons into the Gulf of California and treat it as one thing that can be made predictable and controllable. This paper argues that the Colorado River is a story of ‘colonial becoming,’ the making of a ‘resource’ for purposes of diversion, irrigation, and ultimately dispossession. The paper works first to show how settler-colonialists defined ‘the river,’ then linked it to projects of empire informed by a colonial ontology that saw the desert as bereft of life and needing improvement. I show how these developments led to the ultimate enclosure of the river in the Colorado Compact of 1922, which ignored tribal interests while defining the river for the states. I conclude with a consideration of climate science and the role of physical geographers in reifying colonial divisions in their work and argue that the river is ultimately a product of colonial becoming