THE CERTIFICATED PRIVATE PILOT AND THREE PASSENGERS LANDED AT A REMOTE AREA TO FISH. THE PILOT HAD LANDED AT THE ACCIDENT SITE ON PREVIOUS OCCASIONS. THE LANDING AREA WAS SOFT AND MUDDY WITH HIGH GRASS. A LIGHT RAIN WAS FALLING IN THE AREA. THE PILOT INDICATED THAT DURING THE TAKEOFF ROLL FOR THE RETURN FLIGHT, THE AIRPLANE ACCELERATED TO 60 MPH, BUT LIFTOFF WAS IMPEDED BY HIGH GRASS. THE PILOT ABORTED THE TAKEOFF, AND THE AIRPLANE TRAVELED BEYOND THE TAKEOFF AREA. THE NOSE GEAR THEN FAILED, AND THE AIRPLANE NOSED OVER.