Interesting map of river systems. Weather pattens are more unpredictable than most people realize especially in the mountains. In the Northwest any prediction more than 3 days out is typically pretty iffy unless you are already in the middle of a wet or dry spell.
I wouldn’t say that it is not possible, but there would be major issues trying to get water from Washington to California. First, the Columbia River is in the way, but if you take a look at the topography other more major issues arise. The elevation of the Columbia River is only about 150 ft AGL as it crosses from Eastern Washington to Western Washington. The plateau in Eastern Oregon is over 4000 ft. AGL and if one hoped to channel the water through Western Oregon the passes that one must travel through to get to Northern California reaches a similar elevation.
If one believes that California’s high speed train project is a boondoggle... A 1000 mile long pumping a massive amount of water to a 4000 ft. elevation gain would make it seem like a walk in the park.
Sorry more typos
A 1000 mile long canal and pipelines having to transport a massive amount of water to a 4000 ft. elevation gain would make it seem like a walk in the park.
Much more efficient to go via the sea. They could tap the rivers all up the Oregon and Washington coast line or as this paper looks at go all the way to Alaska or B.C.
https://www.princeton.edu/~ota/disk1/1992/9203/9203.PDF