If things get so bad that Calexit becomes a serious option, is water CA's biggest vulnerability?
California depends greatly on water from out-of-state, like the water supplied by the Colorado River.
Could California really go it alone, or would the US force California to stay by threatening its water supply?
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as well as intakes near Blythe and down near the Mexico border Near Yuma, so they are all on the Arizona / California line.
The rest of the water is either Sierra Nevada Mountains snow-melt or comes from the tributaries of the Sacramento River.
I can't see ANY WAY AT ALL that Trump would be able to order a dam be built just to spite California.
Sorry to be snarky, but put Silicon Valley to work on a Manhattan Project style desalinization solution. This should have been done 30 years ago.
Lots of water out there, just need to sweeten it up.
It's possible to divert it all without damaging the rest of the Interstate Compact. I didn't say easy, because it's going to depend on Colorado and Nevada cooperating. (Arizona and Utah are a given in this scenario.) Here's the map. 
1) About half of California's allotment is pumped over the Continental Divide from the sources of the Arkansas and Platte river on a seasonal basis. (There are times of year when the Front Range uses some Colorado River water; the rest of the year, the Colorado River gets supplemental water from the Arkansas and Platte to keep the flow rate constant.) Turn those pumps off or only flow west to east instead of east to west. What convinces Colorado to do this? Multiple counties in chronic drought conditions. If that water isn't going to the California part of the compact, it can be reallocated to SE and NE Colorado counties, which are hurting right now.
2) Fill Lakes Mead and Powell. Both are well below capacity and the western states need that water storage at capacity. Yes, Powell leaks like a sieve. But that aquifer is tapped in other places, so the water effectively stays in the system.
3) Divert excess flow into the Central Arizona Project Canal to be stored and used by Phoenix Metro and Tucson metro. With more water, Arizona farmers can take up some of the winter produce slack created by cutting off the Imperial Valley.
4) Block the Colorado River Aquaduct and divert remaining flow towards the water deficit Mexico is owed. There will be some minor loss into California on the bed of the Colorado River below Parker, but not a lot. We owe Mexico a LOT of water; they have been more than patient.
It's not impossible. It's not pretty, and it's a devastating scenario, but it's how western water rights work. If California becomes a new entity, they do not inherit their former claims. They cede them and go to the end of the water rights line. (This is why there are Colorado towns with older rights than Mexico, because there are a few places that were founded in 1845-47, and Mexico did not inherit Spain's claims that were defined in the Treaty of Guadalupe Hidalgo in 1848.) Western water rights always work on first in time, first in claim, because they're water rights built for miners, not farmers.
But water may be relatively minor. California could survive on a reduced water budget simply by giving up entirely on alfalfa and soy, reducing rice and greens intended for market (retaining production for local use) and focusing on crops that need the Mediterranean climate.
The two big issues I can see are power -- California has a 30% deficit in power production. They have to fix that immediately, either by getting Oregon and Washington to leave with them, or ramping up solar and wind very fast. And the military.
Twenty-nine Stumps withdraws to Yuma Marine Corps Air Station. And proceeds to use the Imperial Valley for target practice. The naval stations decamp. This is a process that is established, because there's an unlikely but possible scenario involving the San Andreas that would destroy most of the dock space. The east coast and gulf coast dockage will be crowded, but everything can fit. Edwards retreats to Nellis. The smaller bases have their own retreat plans. California probably loses a large portion of their National Guard, too -- they're sworn to the Constitution first and we have already fought this. Secession isn't allowed. That's what 1861-1865 was about.
And California's continental defenses are currently provided by Nevada, Montana and South Dakota based missile systems. So that's gone. They've got a very long land border with no natural barriers, occupied primarily by people who are more likely sympathetic with Il Douche. (Really. Letting the Inland Empire protect the coast's back is a very, very bad idea.) And a long coast with no radar and no surface to air short-range protection. And they're now the 8th biggest economy in the world, with no defenses. Even if the split is as amicable as possible, anyone who wants a toehold in North America now has a perfect target.
Not mentioning that the very idea plays into Putin's greasy mits and it's a good way for half of the progressives in the country to fuck the other half... it's really tactically stupid.
