Speculative fiction writers just love ‘balkanizing’ the US, with enough examples to fill an entire TVTropes page. Usually, the speculated micro-states are divided up according to cultural or historical association (such as a New Confederacy, etc.)
I’ve never really seen any examples using natural geographical boundaries, such as rivers, mountains, and the like which often provided borders for real life countries. So your mission, if you choose to accept it, is to devise a Divided States of America based on natural borders, not just historic or cultural ones.
Assume some conflict or natural disaster which results in fall of US (and most other world) government. The survivors eek out a living for a generation or two, with local leaders taking power. New states are formed. Territory is marked out. Loyalties made. The small militias of these micro-states are going to want boundaries that enable them to defend their territory from all comers.
The whole Cascadia thing in the Northwest would fit the bill. Depending on what particular proposal you’re looking at, you’d have only the parts of Washington and Oregon west of the Cascade Curtain splitting off to join BC, while the remainders of the states can go join Idaho.
The Gulf of Mexico watershed (Mississippi-Missouri-Ohio River watershed, plus basically Texas, Mississippi, Alabama, and parts of Georgia and Florida) might do nicely as Conservative, with two Liberal countries on either side of it. The boundaries would be the Continental Divide to the west, and the Appalachian divide to the east.
The Great Basin (most of Nevada, Utah, etc.) would be part of Conservative, though it’s a separate watershed.
The Great Lakes (St. Lawrence River) watershed would be part of the eastern Liberal country. The boundary between it and Conservative links up with the Appalachian ridge boundary somewhere in north-central Pennsylvania, I believe. Thus, the Hudson and Connecticut River watersheds would be part of eastern Liberal (obviously!).
Carolina - Southeast of the Appalachian Mountains and south of the Tennessee River. Early conflict with Louisiana over control of the mouth of the Mississippi.
New England - St. Lawrence River down along the east of the Ohio River to the Tennessee River. Major regionalism as those in Franklin feel that the Atlantics in the capital of New York are not addressing their concerns.
Ohio - Between the Ohio and North Mississippi River.
Dakota - West of the North Mississippi, north of the Missouri River, east of the Continental Divide.
Louisiana - West of the Mississippi, south of the Missouri River, north of the Red River and east of the Continental Divide. Very rich due to control of the farmlands and Port of New Orleans.
Pacifica - West of the Cascade/Sierra Nevada ranges. The southern section of Califa always threatens to split off at the Siskyou Mountains but cooler heads always prevails realizing that they are extremely powerful together as together they control lumber, food and all of the ports on the west coast. The phrase “comes through Pacifica” is used to mean when someone acts as a middleman and makes a huge profit.
Tejas - Rio Grande to Red River. Mississippi to Continental Divide.
Deseret - The Great Basin. The butt of American jokes since every fringe group and nutter moves there. Mostly left alone by the Federal government the only control is from the capital Salt Lake City and the LDS Church. In the landmark case Bailly v. Deseret, SCOTUS ruled that having a high level member of the LDS Church always elected as Governor does not constitute a theocrasy (1st Amendment). They refused to rule if it was a Republican form of government (Guaranty Clause).
Sonora - Once described as having its only purpose “to fill the gap from Pacifica to Tejas” it grew after the Transcontinental Railroad went through Santa Fe and Flagstaff (You didn’t think it would go through Deseret did you?). Kept its territorial status until quite late until a post-WW1 movement drove Sonora into statehood in 1921.
Montana - Between the Continental Divide and the Cascades. North of the Snake River to Canada. Home of the “Tenth Amendment Movement”.
Upstate New York is mostly already divided on geographic lines. To the north you have the Great Lakes which would be a good line of defense, not to mention the Champlaign Lakes to the Northeast. To the west is Chautauqua Gorge/Chautauqua Lake line which is an obvious natural border if not so defensible.
Then to the East you have the Hudson and to the South you have the Southern Tier Valley. The problem with this is that for most of human history, cultures have inhabited both sides of a river valley in order to take advantage of the water. But still it is natural that these relatively rich places should form boundaries of some sort.
The only place that doesn’t have an obvious border near the actual border is the Catskills. Looking at a map you do have a line of reserviors, but who knows if they’d stay up in a post-breakdown society, and they’re probably useless as a line of defense because it is so easy to infiltrate warriors through hilly forests in a small-army era.