@MushroomGrowing up in Idaho, I saw a lot of that in the history of the state. One I even touched on in one of my stories. A great deal of Idaho was actually founded by Southerners. The city of Atlanta was named when the first news of the Battle of Atlanta reached the area (and it was reported that Confederate forces had won). Most of the Snake River Valley was originally settled by people who migrated from the South before the war began.
The Northerners tended to go to Western Oregon or California. The Southerners tended to settle in Wyoming, Idaho, and Eastern Oregon and Washington. Or Nevada, like the Clemmons' Brothers.
Uh what? I'm from Eastern Idaho and grew up on the Snake River Plain. What you're describing might be close to correct for Southern/Central Idaho(Burley and Twin Falls vicinity) or off towards Boise. But Eastern Idaho is
Mormon Colonization territory, big time. With the exception of Pocatello which was a railroad town created in the 1890's when a Mormon owned railroad came to the area(but quickly became part of Union Pacific). Yes, there were
some southerners among the Mormons but they were more the exception than the rule. 19th Century Mormons were predominately northerners or European immigrants.
The demographics still bear witness to that today: Pocatello, Idaho is 55.4% Mormon as of 2010, the next largest single group would be Catholic at 16.1%. (Pocatello was also a major railroad hub for Union Pacific so its demographics became a little more "metropolitan" than the rest of the area, at least until the end of the Pullman Cars at which point most of the blacks left town, but not before Pocatello elected Thomas L. "Les" Purce to the city council(1973) and then as Mayor in 1976. He'd later go on to be President of Evergreen University in Olympia, Washington from July 2000 until September 2015)
Idaho Falls, Idaho is 62.6% Mormon.
Blackfoot, Idaho is 60% Mormon.
Rexburg, Idaho is 91.6% Mormon. (BYU-Idaho is heavily skewing this one)
Rigby, Idaho is 77.1% Mormon.
St. Anthony, Idaho is 65.7% Mormon.
McCammon, Idaho is 53% Mormon.
Soda Springs, Idaho is 77.2% Mormon.
But sometimes there is a benefit to such an almost monolithic presence of Mormons in the area:
https://youtu.be/EfLMRuXzclU?t=971 (you only need to watch for about 90 seconds from the linked timestamp. Video topic is the Teton Dam collapse in 1976; the time stamp is about recovery/assistance efforts)
Burley is 53.9% Mormon(but Rupert and Heyburn report being 39.5% Mormon, I guess the Mormons settled the south side of the river and everyone else took the north side?)
Twin Falls, Idaho is 25.7% Mormon.
Jerome, Idaho is 22.6% Mormon. (But it also is more the result of the Minidoka Project of the early 20th Century)
Wendell, Idaho is 23.1% Mormon. (Minidoka project)
Bliss, Idaho is 23.1% Mormon. (Minidoka project)
Boise, Idaho is 16.3% Mormon.
And Atlanta, Idaho is up in the Idaho Batholith of Central Idaho, it certainly isn't on the Snake River Plain.
In my story set in Pocatello, it is fictionalized. But when they talk about their ancestors, it is based on actual history. A large extended family that migrated there in the first half of the 1800's from the South. The old farms and ranches there are dying, but until 40 years ago almost all of the owners really were 3rd and 4th cousins of each other, descended from the original 8 or 9 families that settled there.
https://www.intermountainhistories.org/items/show/203
Pocatello was Indian territory up until 1888. No white man was settling there until that point. Parts of the Portneuf River Valley to the north of Pocatello was still part of the Fort Hall Indian Reservation right up until the start of the 20th Century when their borders were redrawn yet again. But you wouldn't be far from the mark in that Pocatello was having a major crises in the 1980's, but it had nothing to do with the farmers and ranchers. It had to do with one of the city's largest employers(a cannery, IIRC) shutting down operations at the WW2 era Naval Ordinance Plant, among a couple other local issues that hit at the same time. Agriculture in that area has done fine, and continues to do fine, that's "Idaho Potato" country with a strong sugar beet market nearby as well. Yes, the market fluctuates, but they're actually reasonably more stable than a lot of other ag markets in the country/world. The biggest problems they have is a combination of family members not being interested in continuing the tradition and the real estate becoming worth more to developers than it is to the farmer or rancher.