On March 28, 2001, about 1300 Alaska standard time, a wheel/ski equipped Cessna 180 airplane, N91303, sustained substantial damage while landing at an off airport site located about 60 miles west of Iliamna, Alaska. The airplane was being operated as a visual flight rules (VFR) personal flight under Title 14, CFR Part 91, when the accident occurred. The certificated airline transport pilot, and the one passenger aboard, were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated about 1200, from the Iliamna Airport, Iliamna. During a telephone conversation with the National Transportation Safety Board investigator-in-charge on April 11, the pilot reported that he was landing to the northeast on a frozen slough. He said that after touchdown, he noted a football-sized clump of dirt protruding out of the ice in front of the airplane. He said that in an attempt to avoid the dirt clump, he applied full left rudder. He said that the nose of the airplane moved about 20 degrees to the left, but the ground track of the airplane continued in a straight line, and the right ski struck the dirt clump. The right main landing gear collapsed, and the right wing struck the surface of the frozen slough. The airplane sustained substantial damage to the fuselage, right wing leading edge and right wing main spar. The pilot indicated that there were no preaccident mechanical anomalies with the airplane.