WITNESSES SAID THAT THEY OBSERVED TWO AIRCRAFT FLYING ABOUT 3 FEET OVER THE FROZEN, SNOW COVERED RIVER. ONE OF THE AIRCRAFT SKIMMED THE SNOW COVERED SURFACE WITH ITS MAIN GEAR, THEN NOSED OVER. THE ACFT LEFT TRACKS IN THE SNOW, ABOUT 360 FEET PRIOR TO THE NOSE OVER. POST ACCIDENT INVESTIGATION REVEALED NO MECHANICAL MALFUNCTION OR FUEL SHORTAGE.