I've been seeing these all my life and for some reason never thought to ask if anyone else noticed them. I never found them particularly distracting and didn't know they were a sign of bad typography. Still, cool to see an article about them.
Relevant xkcd: http://xkcd.com/1015/.
Rivers stand out more in blurred text, even in central vision.
Some proofreaders would note them should they find / notice any but some wouldn't.
Now that I think of it, using OP's findings it would probably wouldn't really be too hard to write a "plugin" taking screenshots of Quark XPress / Adobe InDesign / LaTeX preview and showing "rivers" in a HUD on top of the screen-rendered pages. This would make chasing "rivers" quite easy.