The pilot was departing a remote landing area in a tundra tire equipped airplane. He had visited the area many times in the past. The takeoff area was about 800 feet long and contained some areas of about 3-feet high grass. The takeoff area also contained areas of soft terrain and water. During the takeoff run, the airplane became airborne momentarily and then settled back to the surface. The right main landing gear tire struck a stump and the left wing contacted the ground. The airplane then nosed down in the soft terrain.