OOC: Wouldn't this cause border issues with California, though(and, by extension, *Arizona as well)? I think it'd work out much better if we just kept the OTL western border, although everything else should be fine.
OOC: It wouldn't be too far west, just more of a even split down the Sierra Nevadas rather than California getting the lion's share.
Here is what I was thinking. Gold was discovered in northern California before the OTL discovery at Sutter's Mill but earlier. 1830's maybe? This still brought a lot of population to California as in OTL and helped San Francisco and Sacramento develop. Pretty sure the Sacramento River could be used for trade. This would lead to a scenario similar to Texas where they were too far from Mexico to make them follow the requirements to emigrate and too far to successfully put down a rebellion.
They claimed most of OTL's California Republic but also claimed a bit further north into parts of Oregon. I figure with the US having British Columbia there is a lot more room for divergent state borders in the Northwest. When California joined the US they tried to get all their claims recognized, but the US wanted to make states out of TTL's Mexican Cession. Since New Mexico could resist not being part of Texas, the people in the east parts of California tried the same. The US didn't want California too powerful of a California to not upset the balance in Congress, so they gave them a choice to either back down on their claims east of the Sierra Nevadas or to back down on their claims in the Oregon area. (Let's say somewhere around Crater Lake?)
So it came down to either a fight for the mountains (where the gold hadn't been discovered yet) or more coastal and farmland. It was a fairly easy decision. Also, Pah-Ute county split among TTL's Southern Californian state and Arizona, with Arizona going a bit further north to have more of the Colorado River.
TL;DR, basically OTL southern Nevada is part of one of the Californian states and Arizona, with the Northern Californian state (i.e. mega-State of Jefferson [maybe the state named after Benedict Arnold?]), going a bit further north. And the more I think about it, three Californias, while cool sounding, seems a bit too complicated, and given you're obvious love of the place two sounds a much better solution. Not trying for a California-screw, trust me, just trying to have different state borders in the West. Too many TL's keep them the same.
Edit: Did we ever talk about Baja California? I know its a minor-cliche to give it to the US but I'm all for it. Maybe the border is further south so the US has access to the Gulf of California/Sea of Cortez?