On August 20, 2000, approximately 1540 Pacific daylight time, a Cessna 172B, N8178X, nosed over during a precautionary landing in a marsh about one-half mile north of the departure end of runway 36 at Sunriver Airport, Sunriver, Oregon. The private pilot and his four passengers were not injured, but the aircraft, which was owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which had just departed Sunriver Airport en route to Evergreen Field, Vancouver, Washington, was being operated in visual meteorological conditions. The ELT, which was activated by the accident, was turned off at the scene. No flight plan had been filed. According to the pilot, during the takeoff ground roll, when the airspeed reached 60 mph, he rotated the aircraft and it lifted off with the stall warning horn sounding. After reaching an altitude of about 10 feet above the ground, with the horn continuing to sound, the aircraft did not continue to climb or accelerate. The pilot therefore lowered the nose and allowed the aircraft to settle back to the runway. At that point, although the airspeed indicator was showing 45 mph, the pilot decided to continue the takeoff. When the airspeed reached 65 mph, he rotated a second time ("more slowly"). As the aircraft neared the end of the runway, it lifted off and slowly began to climb. Approximately 200 feet past the end of the runway, the aircraft encountered a downdraft and the pilot "...pulled the nose up." As the nose was raised, the stall warning horn sounded, and the pilot once again lowered the nose. At that point, the aircraft, which was then about 60 to 80 feet above the ground, encountered another downdraft. Almost immediately after encountering this second downdraft, the pilot decided that there was "...a substantial risk of encountering tall trees approximately 1/4 mile ahead." He therefore made the decision to execute a precautionary landing straight ahead. Although he made a successful touchdown on the wet, marshy/swampy terrain, when the nose wheel touched down, it dug into the soft mud and the aircraft nosed over, The terrain elevation was about 4,200 feet above sea level (MSL), the temperature was about 68 degrees Fahrenheit, and the barometric pressure at the nearest reporting point (Redmond, Oregon) was 30.18 inches of Mercury. Based on these parameters, the density altitude was determined to be approximately 5,550 feet. The pilot, who did not perform takeoff performance calculations prior to the attempted departure, said that he did not believe there had been any problems with the engine or the aircraft systems.