During a flight to go fishing on a river, two of the three planned landing strips the pilot was going to land on were covered with water. The third had fog present. As a result, the pilot looked for an alternate place to land and discovered a gravel bar in the river which looked to be about 800 feet long. He flew several passes over the gravel bar to assess its condition and then decided to land. During the landing flare, he noticed that the airplane was below the edge of the gravel. He added power to try and clear the edge, but the airplane touched down on a 1 to 2 foot long muddy, downwash area that existed just prior to the gravel bar, and the left main landing gear then struck the gravel bar's edge. Examination of the airplane revealed that it had incurred substantial damage to the airframe. The pilot reported there were no preaccident mechanical malfunctions or anomalies that would have precluded normal operation.