It's touched on in the article, but the science behind sticking to the stream bottom is covered by this:
https://en.wikipedia.org/wiki/Law_of_the_wall
If you're not mathematically inclined, this boils down to there being a logarithmic relationship between stream velocity and distance from the stream bed (the wall). While water might be ripping along at the top of the stream, where you can see it, near the stream bed, it is barely moving. It's a safe place to hang out, just like being downstream of a rock, or close to a bank. Of course, if the fish moves out of the low flow area, then it could potentially be washed downstream, until it either reaches another low flow area, or it succumbs.
More than "nothing" happens to them. The risk for injury or death certainly goes up, but the majority of them survive. Their habitat post-flood might be vastly different, though.