On August 19, 2006, about 1015 Alaska daylight time, a float-equipped, Piper PA-16 airplane, N5280H, sustained substantial damage when it collided with trees following a loss of engine power during initial climb after takeoff from Mirror Lake, about 3 miles northwest of Chugiak, Alaska. The airplane was being operated by the pilot as a visual flight rules (VFR) personal cross-country flight under Title 14, CFR Part 91, when the accident occurred. The private certificated pilot received serious injuries, and the sole passenger received minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was bound for Katie Lake, near Talkeetna, Alaska. During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on September 5, the pilot said he'd moored the float-equipped airplane in a slip at the lake about a week before the accident. He reported that when he parked the airplane, both wing fuel tanks were about one-quarter full, and that the morning of the accident he fueled both tanks with a mixture of aviation fuel and auto fuel. He said there had been heavy rains at the lake the week before the accident. The pilot reported checking the fuselage and gascolator fuel drains for contaminants before the accident flight. He said on the day of the accident, the ceiling at the lake was about 3000 feet, and there was light rain and mist. He indicated the preflight and run-up were normal, and that he used carburetor heat when the engine was idling, but not during step-taxi or takeoff. He step-taxied the airplane to the opposite end of the lake, and reported that when he reduced engine power, the engine backfired. He said he step-taxied across the lake, and everything appeared normal, so he initiated a takeoff. He stated that the airplane lifted off the water at 50 miles per hour, and that the attitude and climb appeared normal, however he felt the airplane's engine was not producing as much power as usual. As the airplane climbed to about 150 feet above lake level, and over trees along the shore, he said the engine backfired twice, and quit completely. He started a turn toward the lake, but the airplane stalled, and descended into trees. An on-site inspection of the airplane on August 19, revealed the airplane standing vertically on its nose and floats in a heavily wooded area. The forward section of the floats were bent upward 90 degrees, parallel to the ground. Control continuity was established for all the flight control surfaces. The fuel selector was in the "on" position, and both wing fuel tanks contained fuel. The carburetor heat selector was in the "off" position, and the throttle control was in the full throttle position. The mixture control was in the full-rich position, the primer was in and locked. The magneto selector was in the "off" position, and the key had been removed. The fuselage and wings had structural damage. The engine was not accessible for examination at the accident site. At the site, the IIC located 5-6 pieces of "chopped" wood. Each piece was about 18 inches long, about 2 inches in diameter, and gave the appearance of having been cut with the airplane's propeller during the descent. The airplane was disassembled by friends of the pilot and stored. An automated weather observation taken at the Birchwood Airport at 1036, about 2 miles from the lake, reported few clouds at 3600 feet, few clouds at 4400 feet, and a broken layer of clouds at 7000 feet, a temperature of 52 degrees F, and a dew point of 45 degrees F. In a written statement to the NTSB, the pilot wrote that with the airplane on floats, the wings were in a more level attitude than when sitting on the tailwheel, and any water in the tanks was more likely to move away from the sumps. He added that his practice of standing on the front of the floats while taking the fuel samples probably moved any contaminants further from the sumps. He reported that in his opinion the loss of power was due to water contamination of the fuel as a result of leaving the tanks three-quarters empty during a rainy period, in conjunction with a flawed sumping procedure.