The commercial pilot attempted to takeoff downstream on a 14 CFR Part 91 business flight in a float equipped airplane. He stated that as he reached his predetermined point to abort the takeoff if not airborne, he continued the takeoff run because he felt the airplane would liftoff soon. The airplane did not become airborne, and while attempting to negotiate a sharp turn in the river while on-step, the airplane collided with alders and the left riverbank, sustaining structural damage to the wings, fuselage, and floats.