There is some interesting discussion on the LaTeX stack overflow page about the challenge of detecting and preventing rivers during the typesetting process:
Avoiding rivers becomes a rather non-trivial optimization problem. In Holkner's paper he found that it took ~1 minute just to typeset 1200 words. Some of his experiments took more than six hours to complete.
https://tex.stackexchange.com/questions/4507/avoiding-rivers...
https://tex.stackexchange.com/questions/29049/how-to-define-...
And if you really want to get into it, there is a rather detailed paper by Alex Holkner: https://gwern.net/doc/design/typography/tex/2006-holkner.pdf
Avoiding rivers becomes a rather non-trivial optimization problem. In Holkner's paper he found that it took ~1 minute just to typeset 1200 words. Some of his experiments took more than six hours to complete.