Well, a subject I know well. Ok, I saw you want something that will handle medium to big rivers. Now, how much do you wish to spend max and do you want a one man or multi man boat? Do you want to run any whitewater?
There are many topics on this if you search back. You can talk with Osprey about cats too (those of us who run bigger/hardcore boats call them cats). The pontoon boats listed above are good if you want a simple vessel to take you from fishing spot to fishing spot. They aren't something you actually want to fish out of. I've tried years ago out of my old Outcast, but could never effectively (and safely) fish from it. I usually had to pull off somewhere and fish.
Now, here's what I have, a 9ft steelheader for my one man floats, and a 16ft Aire for my multiperson floats (my driftboat you could say). Both are whitewater rated and can handle a lot of weight. If you want a boat that's multipurpose and will hold you for years on end, go up to at least a 12fter. It's a good one man boat, and you can handle 2 people if you want. My 16fter can safely handle 4 people fishing. It's better to buy a boat that's a little big then to buy one that's automatically too small. My 9 fter is great, but at times it's a bit small roomwise with all the gear and such. If you have any questions, email me and I'll see what I can do for you.
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you haven't lived til you've rowed a cataraft. Friends don't let friends run Outcasts.