As for the Thunderbird account, nothing seems terribly out of the ordinary, except perhaps for the wingspan, which may either be chalked up to exaggeration, or instead refer to a rather narrow section of the river. If Thunderbirds are Teratorns, their wingspan could match your average hang-glider, and that’s pretty impressive right there. As for the sound, well, there’s a reason they were called thunderbirds, you know. Many accounts report loud wingbeats, although I’ll admit you’d have to infer that from the article, as it makes it sound as if it were constant.