A few years ago I strapped on the tanks and took a dip in Willow Creek, down where it dumps into the Susitna. I noticed that there were a LOT of salmon in there (it was on a weekday when nobody was fishing), but what really caught my attention was that many of the fish would lay in holes along the bottom. It didn't have to be much of a hole either; just a couple inches deeper than the surrounding area was enough. I believe the holes offer less resistance to the fish as they swim upstream, therefore these areas collect fish.
I believe this is the reason why some dipnetters have better success than others. Fish the holes when you can find them, and your chances for success go way up.
I know many of you already know this, but I thought it was interesting to actually see it happening underwater.