On June 28, 1997, about 1700 Alaska daylight time, a float equipped Helio H295 airplane, N6343V, sustained substantial damage during takeoff from a remote, off airport site on the Nushagak River, located about six miles southwest of Ekwok, Alaska. The private pilot and three passengers aboard were not injured. The Title 14 CFR Part 91 personal flight operated in visual meteorological conditions. A VFR flight plan was filed. The flight was departing for Lake Iliamna, Iliamna, Alaska. During a telephone conversation with the NTSB investigator-in-charge on June 28, the pilot related he was attempting to takeoff down river. He estimated the temperature as approximately 70 degrees F, and the wind to be from the south at 10 knots. Shortly after becoming airborne, he said he believes the engine lost partial power, and the airplane settled onto the river. Shortly after touchdown, the right, and then the left wing, collided with trees and brush along the shoreline. The pilot and passengers were able to exit the airplane unaided. In the pilot's report submitted to the NTSB, he notes the wind as from the north at 5 knots, and the temperature as approximately 80 degrees F. He wrote, in part, "The aircraft was a little slow to come off the step and took about 400 yards to take off. I lifted the right float, and then the left float at 60 mph indicated. The aircraft rose to about 3 feet and accelerated to just over 60 mph, but would not accelerate further. I pulled my eyes inside and checked manifold pressure, rpm, mag setting, flaps; all were okay. As my eyes came back outside only 3 to 5 seconds the airplane mushed back toward the water. Airspeed had deteriorated to about 55 mph and the airplane touched back down on the water. Immediately in front of the airplane was one lobe of the gravel bar. With the prospect of hitting the gravel bar at 50 mph and the engine still running smoothly but at inadequate power for flight I choose to step turn around the right side of the gravel bar into a deep channel on the back side. As I cleared the gravel bar I noticed the roots of a stump sticking up through the water. I had to jog to the right to miss it, pushing me too close to the right river bank. I noticed a...tree overhanging the water...the right wing hit the tree about 12 inches from the tip...and spun it nose first into the steep bank." Another conversation with the pilot after the airplane was recovered, and prior to inspection of the airplane by an FAA inspector, disclosed the pilot believed that the partial loss of engine power could have been caused by a leak in the diaphragm of the pressure type carburetor. The airplane's engine was inspected by an FAA airworthiness inspector from the Anchorage Flight Standards District Office. The inspector discovered no evidence of any preimpact mechanical anomaly. The pressure carburetor was removed, and taken to a repair facility for inspection and flow testing. No mechanical deficiencies were noted, and the carburetor performed within acceptable factory specified tolerances.