The pilot reported he was attempting to takeoff from a remote gravel bar. He noted the wind conditions as generally calm, with infrequent gusts. Shortly after liftoff, the airplane settled onto the gravel bar and collided with rocks, collapsing the main landing gear. The airplane was consumed by a postcrash fire. The pilot said there were no preimpact mechanical problems with the airplane, and that a wind shift or gust may have contributed to the accident.