The pilot was taxiing along the river at a speed of about 35 knots. At the end of the taxi run he initiated a turn to line up for take-off by applying full power and left rudder while holding the control column back. As the aircraft began to turn, the pilot sensed that the aircraft felt unusual. He closed the throttle, and noticed that the left pontoon float had separated from the wing and was being dragged by the fuel line. The water rudder was lowered and the pilot steered the aircraft towards the bank of the river. However, while concentrating on the left wing, the pilot did not notice that the right wing had insufficient clearance from trees on the river bank, and a collision ensued.