It depends on what your talking about, Sid.
Any sea kayak will work in a river if the water doesn't get too verticle. If you're talking about one of those 8' jellybeans that can do cartwheels and jump 20' waterfalls then you'd better find the polyethylene web site. Nobody who builds boats for themselves is going to treat their boat like a carnival ride and turn it into a pile of sticks.
Charlie