The private pilot was making a local, personal flight under Title 14, CFR Part 91. He said he decided to attempt a landing on a snow covered sandbar. He reported that his intent was to lay down tracks to test the firmness of the surface prior to landing. He said during the process the airplane's main landing gear broke through the crusted snow, and the airplane nosed over. The pilot said there were no mechanical problems with the airplane prior to the accident.