A witness reported that he was outside when he saw the accident airplane maneuvering about 30 to 50 ft above the ground along a river. He remarked that the airplane seemed low. The witness said that the engine sounded like it was operating normally and the airplane was not flying erratically. The witness saw the airplane bank and then it struck a powerline. There was a bright flash of light, the airplane disappeared from his view, and then he heard a loud bang. The witness went to the other side of his property and saw that the airplane was upside down on a sandbar along the river. He estimated that the powerline was about 40 to 45 ft above ground. The pilot was seriously injured, and the airplane sustained substantial damage to the fuselage, wing struts, and engine mount. The investigator in charge made several attempts to obtain a statement from the pilot. At the time this report was written, the pilot had not provided a statement pertaining to the accident.