COMBS AIRWAYS, FLT 709, COLLIDED WITH A SNOW BANK DURING THE LANDING ROLL AT SWEETWATER COUNTY APRT. THE CAPT REPORTED THAT 'IMMEDIATELY AFTER TOUCHDOWN, THE ACFT VEERED LEFT, STRUCK A SNOW BANK, THEN VEERED TO THE RIGHT 180 DEG COMING TO REST ON THE RWY FACING TO THE EAST.' THE PROP & GEAR BOX SEPARATED FROM THE RIGHT ENG. AN INVESTIGATION REVEALED NO EVIDENCE OF A MALFUNCTION OR FAILURE PRIOR TO THE ACDNT. THE ARPT MANAGER REPORTED THAT SNOW SHOWERS & DRIFTING SNOW HAD ALMOST TOTALLY COVERED THE RWY MARKINGS, INCLUDING THE RWY CENTERLINE. SNOW PLOWING OPERATIONS HAD BEEN IN EFFECT ON THE DAY OF THE ACDNT. THE CENTER OF THE PLOWED AREA WAS SEVERAL FEET TO THE RIGHT OF THE RWY CENTERLINE. NUMEROUS FINGER DRIFTS WERE PRESENT FROM THE LEFT EDGE OF THE PLOWED AREA TO THE CENTER OF THE RWY. THERE WERE 2 FT SNOW BANKS ON EACH SIDE OF THE SNOW PLOWED AREA.