THE PILOT-IN-COMMAND STATED THAT SHE ENCOUNTERED A DOWNDRAFT ON SHORT FINAL. THE AIRPLANE FIRST TOUCHED DOWN ABOUT 4 FEET SHORT OF THE RUNWAY, BOUNCED, THEN LANDED HARD ON THE RUNWAY CAUSING ALL LANDING GEAR TO SEPARATE FROM THE FUSELAGE. THE POINT OF FIRST TOUCHDOWN WAS SLOPED DOWN AND AWAY FROM THE RUNWAY ABOUT 30 DEGREES. THE END OF THE RUNWAY WAS ABOUT 100 FEET FROM A RIVER BANK WHICH DROPPED RAPIDLY ABOUT 20 FEET.