I think the vision mode that detects rivers is more of a "big-picture, perhriphery" type where the vision mode that you use for for reading is a focused, small area type. I think you will always catch them in the big picture, but may not notice when reading.

Definitely. People typesetting books are supposed to get rid of them but it's a manual process. As I've written in another comment I'd typically "blur" my vision a bit and quickly scan all the pages of the book to track these down (on screen) and then remove them. Don't know what was the technique of other typesetters when it came down to finding these.

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.