Same goal, different solution
This 'categorization' problem is a common issue for sites like this ... Do you group by topic? Do you group by geographical location? How many forums do you create? How many nested levels of forums do you create? And so on. If you get carried away, you'll end up having too many fragmented forums and the site will become too cumbersome to use. (Heck, I'm a fairly new member, and I've long since given up on going to each forum to read postings. I only use the 'New Posts' link now.)
Being a software guy, here is my $0.02 ...
One way to address this problem is the capability to add 'flags' (or attributes) to our postings. For example, if I were to write about a hike in NH, I'd flag it as 'White Mountains' and post it to Trip Reports. If I were to write about a hike in upstate NY, I'd flag it as 'Adirondacks' and post it to Trip Reports. Same forum. All the postings are still together. The postings just have different flags.
Once you have flags, then you just need the ability to filter postings based on these flags. To see only Trip Report postings for the White Mountains, a user just needs to click the the 'White Mountains' flag, and viola! Only White Mountains postings are displayed. The Trip Reports forum is just an example. It would apply to other forums too.
To avoid confusion, you might want to limit the setting of flags to only the first posting that starts a new thread. You could also simplify things by letting users select which flag(s) they wanted their postings to have by default (in their user profile).
I expect the vBulletin software that powers these forums provides powerful tools/scripting to implement this flag/filter capability, but the 'proof of concept' version could just be a glorified version of 'Search' -- Simple 'Flag' buttons to add special text to a posting, and simple 'Filter' buttons to search for that text.
-BriFly