On February 21, 2004, about 1655 central standard time, a ski-equipped Cessna 120, N73034, operated by a private pilot, sustained substantial damage when it impacted uneven terrain and subsequently nosed over during a landing on the frozen Wisconsin River, near Merrill, Wisconsin. The personal flight was operated under 14 CFR part 91. Visual meteorological conditions prevailed at the time of the accident. The pilot and passenger reported no injuries. The flight originated from the Merrill Municipal Airport (RRL), near Merrill, Wisconsin, about 1600. The pilot reported: The accident occurred when I landed on Lake Alexander, a portion of the Wisconsin River. I had determined incorrectly that this was a very good place to land. The snow was fresh, smooth, evenly colored, and not tracked up by vehicle traffic. Upon landing my [right] ski fell into a snowmobile path that was [approximately] 18 [inches] deep. The rut was at a near 45 [degrees] to my direction. The ski buried and dove stopping suddenly flipping me over. ... The light was 'flat' making surface detail difficult to [perceive]. At 1657, the recorded RRL weather was: Winds 280 degrees at 6 knots; visibility 10 statute miles; sky condition clear; temperature -2 degrees C; dew point -8 degrees C; altimeter 29.94 inches of mercury. The pilot reported no mechanical malfunctions in reference to the flight.