The commercial certificated pilot was landing on a remote gravel bar on a pleasure flight, under Title 14, CFR Part 91. He reported that upon landing the airplane hit either a hole, a rock, or a stump, which he did not see. He stated the airplane was diverted from its intended landing path, and the wing and propeller struck other objects before the airplane came to rest in the river. He said the wing, lift-strut, and propeller were damaged.