A little bit of movie magic. I love the effect it gives though...just floating over the waterfall!
I recorded the sound of the waterfall separately from the flight, then overdubbed the audio in post production. I then faded the audio in or out,depending on what was going on in the video.
The audio was recorded using my iPhone "Voice Memos" app that comes with the iPhone. As a backup, I planned to just use sound effects off the internet if I had to. Fortunately, the quality wasn't that bad, and I went ahead and used it. I had never tried this technique before, so it was a bit of an experiment for me. The only issue I ran into, was that the audio was mono, and my software only played the audio on my left speaker. It took me a bit of experimentation to figure out how to play the audio in stereo.