The non-certificated pilot stated that he was making low passes over a river while looking down at people who were on the river. After looking up, the pilot noticed that he was approaching the tree tops, added engine power, and raised the airplane's nose in order to avoid the trees. He stated that the airplane stalled and impacted the tree tops, after which it nosed over and fell about 50 feet to the ground. A Federal Aviation Administration inspector, who examined the airplane after the accident, stated that the airplane sustained substantial damage to the wings and fuselage. The pilot reported that there were no mechanical malfunctions or anomalies with the airplane.