How much weight is too much on your back while MTB'ing?

[stevage]

For such a short distance, why not try something simple (like a backpack), then spend money to fix problems you encounter? Strap some stuff to your handlebars (eg, the boat and paddle, like eric does), some to a rear rack (you can get much cheaper than OMM), the rest in a backpack...

[wdlandparker]

i would recommend having a larger pack(~2000 cubic inches) and a rack, with all of the stuff that you use for both riding and rafting in the pack, then put all  the heavy raft gear on a rack. then you have the option to put all the stuff in/on the pack in the occasion that your rack was to fail or if you have to bushwhack with the bike. thats just my opinion though

[longman]

I have a Bob and racks and both have their benefits and drawbacks. I rode the Divide route plus sidetrips this summer and used Old Man Mountain Racks, I had zero bike issues or rack issues, heck I didn't even get a flat in ~4000 miles. The racks were bomber and the bike handled fantastic, I only had to walk two little uphill sections on the whole route (Fleecer Ridge and a tiny section of the Lava Mountain Trail) which were unrideable even without a load, I rode everything else and some of it is techy or steep, I hit 53mph on a downhill and rode it like a mountain bike all the time. I saw lots of trailers being used by other folks but also lots of triple tire tracks with footprints next to them when the climbs got steep or techy. I wouldn't hesitate for a second to recommend the racks to anyone who needs to carry a lot of stuff, I had my clothes, food, cooking gear and 11 litres of water on my rear rack for the Great Divide Basin and still no problems.
 When I use the trailer, mainly for groceries, it's always noticeable... I have to slow right down for kerbs and steps (down as well as up) otherwise it bounces a lot and stuff gets bent, it's less stable at high speed than racks and panniers but it's so convenient to throw stuff in there, especially big packs of loo roll, boxes of cereal etc, even other bikes! The Bob is really easy to attach and remove and switch to many bikes, I just keep a Bob QR in each bike and it takes a minute tops, whereas the racks need tools and take a lot longer.
You can't go wrong either way but if the raft and kit is bulky and you don't mind riding a bit slower go for the Bob.