THE PILOT STATED HIS APPROACH AND LANDING SPEED WAS TOO FAST AND THE AIRPLANE BOUNCED UPON TOUCHDOWN. WHEN IT TOUCHED DOWN A SECOND TIME THE LEFT MAIN GEAR STRUCK A SMALL PILE OF SAND AND STICKS. TO REGAIN DIRECTIONAL CONTROL THE PILOT ADDED FULL POWER. HE STATED HE HAD INSUFFICIENT ROOM IN WHICH TO STOP THE AIRPLANE SO HE ATTEMPTED TO ACCELERATE AND CLIMB OUT. THE AIRPLANE'S MAIN GEAR STRUCK SOME BRUSH LOCATED 75 FEET PAST THE DEPARTURE END OF THE LANDING/TAKEOFF AREA. THE AIRPLANE NOSED OVER INTO THE MULCHATNA RIVER.