I would suggest you use Thread Prefixes for this instead of sub-forums. So, create a set of Thread Prefixes (General, Idaho, North Dakota, South Dakota) and make them required in that forum (Forum Manager > Require Threads to have a Prefix > Yes). Then, install a modification like this one -
Make Prefixes Clickable to Filter Forumdisplay - so that users may click on a prefix and only see threads with that specific prefix.