The good side of the flooding if it ever does in reality happen in California, is that the State of Jefferson would have a natural border to the south, and to the North, cause I am pretty sure the willamette valley in Oregon would also be flooded, during such a catastrophe, Thus making the northern and southern mountain men of their ex-states rather happy.
did the willamette valley flood the last time?
Copy and paste from google search wiki-post:
Yeah it floods, sometimes real bad.
aren't valley's created by rivers?
That is one way valleys are created, another is by Glacial retreation. I am not sure how old the willamette valley is. I think also valleys, maybe smaller ones can be created by pyroclastic flows, and then heavy rains washing the muds and ashes and scouring the lands. But not real sure on that. But the retreating glaciers do cause valleys. Most of the U shaped valleys were from Glaciers.