Where is the water coming from? It’s not coming from the Great Lakes or the Mississippi River. Where are these enormous volumes of untapped water that we can pump over the Rocky Mountains?
For comparison, the Phoenix metro area uses 2.3 million acre feet of water per year (40% of which is used in agriculture).[2] So a bit over 1% of the Columbia's annual flow.
Another comparison: the Colorado "in its natural state" averaged just 22,500 cfs, less than 10% of the Columbia.[3]
I'm not saying that piping water from Oregon to the southwest is a good idea, but it's not as if we don't have water in the west. It's just a lot further north than most of the population centers.
[1] https://en.wikipedia.org/wiki/Columbia_River#Discharge [2] https://askabiologist.asu.edu/questions/how-much-water-are-w... [3] https://en.wikipedia.org/wiki/Colorado_River#Discharge