Often, these things only <b>seem</b> obvious in hindsight.  After all, as you pointed out, this can only be done under the right conditions, and not on all rivers. So "obviously," if you were next to a river that didn't have the right conditions, you wouldn't think of this, or if you did and tried it, it wouldn't work, or at least it wouldn't work very well.<br><br>Of course, if such ideas are not really so obvious, there are probably many such ideas remaining to be discovered by clever or patient (trial and error) people.  Or people who stumble upon them by mere luck.