On April 4, 1998, about 1300 Alaska standard time, a ski equipped Piper PA-18 airplane, N4320Z, sustained substantial damage while landing on a remote river gravel bar, about 40 miles northeast of Talkeetna, Alaska. The airplane was being operated as a visual flight rules (VFR) local area personal flight when the accident occurred. The airplane is registered to, and operated by the pilot. The certificated commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. A VFR flight plan was filed. The flight originated at the Talkeetna Airport, about 1200. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on April 7, 1998, at 1320, the pilot reported he was landing on a snow covered gravel bar. During the landing roll, the landing gear skis struck an object buried in the snow. The pilot added power, and departed without stopping. The pilot did not observe what type of object was in the snow. The pilot returned to Talkeetna, and discovered damage to the fuselage at the right main landing gear attach point.