THE PLT RPRTD THAT AS THE ACFT WAS ON A SHORT FINAL APCH TO LAND ON A GRAVEL BAR, IT ENCOUNTERED A DOWNDRAFT. TO AVOID CONTACTING THE WATER, HE CORRECTED WITH ELEVATOR CONTROL & POWER, BUT THE ACFT GAINED MORE ALTITUDE THAN HE DESIRED. SUBSEQUENTLY, IT TOUCHED DOWN ON THE GRAVEL BAR BEYOND THE INTENDED TOUCHDOWN POINT. AS THE PLT WAS APPLYING HEAVY BRAKING TO STOP ON THE GRAVEL BAR, THE WHEELS DUG INTO THE SANDY TERRAIN & THE ACFT NOSED OVER.