I wasn't confused. I was wrong. I have an old TeX reference which suggests TeX pays attention to rivers. But based on your challenge, I skimmed through the d

I wasn't confused. I was wrong. I have an old TeX reference which suggests TeX pays attention to rivers. But based on your challenge, I skimmed through the description of the line-break algorithm in Knuth's "The TeXbook," and found nothing about rivers. (There is a bit of attention paid to making sure adjacent lines are compatible in terms of how squashed or spread out they are, but apparently nothing that tries to catch rivers.)