That's a problem I'd love to have. I think the best way to keep a site like this going is through donations, but honestly have no idea whether that is feasible or not.
I really don't have problem with ads. As much as I love building things, there is a definite opportunity cost that comes with spending time in front of the computer that could be spent, you know, actually hiking with my family.
The overall design looks very nice so Google adsense type ads would take away from that.
Have you thought about getting individual trails "sponsored" by a local outfitter? In the info box on the left side you could have Sponsored by Joes Outfitters with a link to their site, and it would match the rest of the text so it wouldn't even be that noticeable.
Here's my concern with ads. Even if I managed a 2% CTR with a subtle ad, I'd still have 98% of my users carefully avoiding a part of the page that could be devoted to something else.
Someone mentioned affiliate links in a bibliography section. I could see that working because it's not intrusive, but you'd have to be careful not letting your sources be dictated by what's available on Amazon. I like donations because it keeps the financial goals of the site inline with creating the best experience.
I could see sponsorship links working, maybe on the homepage, or maybe in a separate /sponsors page. I'm not completely sure, it's a tricky problem.
