On July 17, 2000, about 0845 Alaska daylight time, a float equipped DeHavilland DHC-2 airplane, N345KA, sustained substantial damage during takeoff when it collided with a tree on Alexander Creek, about 15 miles west of Big Lake, Alaska, at 61 degrees, 36 minutes north latitude, 150 degrees, 40 minutes west longitude. The solo airline transport pilot was not injured. The positioning flight was being conducted under 14 CFR Part 91, by Ketchum Air Service, Inc., Anchorage, Alaska. The flight was returning to Lake Hood Seaplane Base, Anchorage, after dropping passengers at the company lodge on Alexander Creek. Visual meteorological conditions prevailed at the time of the accident, and a company VFR flight plan was filed. During a telephone interview with the NTSB investigator-in-charge on July 20, the pilot stated he lost directional control of the airplane during the initial takeoff run. He indicated the airplane was just coming up on step, and he was taking off upstream, with an eight knot quartering tailwind from the left. He said the airplane swerved left, and the left wing struck a tree on the river bank. Postaccident inspection of the airplane by the NTSB investigator-in-charge on July 18, revealed the first rib inboard of the landing light was crushed aft to the left wing main spar.