So it appears to be going by county, so let's get a map:
Basically, the vast majority of the land just went back in time.
Upstream, US is hosed on a grand scale. It just lost half its population, and that's the specific half that does the lion's share of fighting, food production, mining, and basically all the resource extraction. Even in California, the farmlands in the Central Valley went to Trump so they just lost all that production. They're going to have to recolonize and unfortunately, they have only the inner-city population to do it with. Basically you've got a lot of Twitter employees and LA gangs who are going to have to make a go of being pioneers, it won't be pretty.
California in particular has just become a death world. The various canal systems to bring in water have huge sections missing so the densely populated west coast is still present, but not enough water to keep them alive. Abundant supplies of meat and grain from the midwest and south are also nowhere to be found, they'll be starving within a few weeks, possibly only days. They have to get the people out, no way around it. Fortunately, the ports are still intact (planes just don't have the capacity and convenient airports like Oklahoma's international one are gone) but even that's going to be able to handle the millions of suddenly-thirsty Californians fleeing the death zone.
Mexico's about to get refugee trains from California charging across its borders.
The upstream US just got a mountain of natural resources so they can put things back together but it's going to hurt, and there's every chance other nations are going to be looking to take a bite out of those resources. The US navy is still intact since those ships were mostly not on land, so they can prevent, say, China or Russia from gouging out the coasts but only if the remaining bits of the population don't vote to go ahead and give it to them. Mexico is in a position to take back some land if they choose to.