In the future I imagine your question about the river pollution is almost nonsensical. That is if we manage to actually get there. Going by current news about the future of earth's ecology we might not actually make it much further. Gotta admit though heroic human optimizations seemed like they got us pretty far but the lag time in reacting to things is a bit too long when humans are in the loop. So the sooner they are out of that critical loop the better.
Speaking of which: http://www.sciencealert.com/photos-reveal-more-than-200-brig...
Some more examples:
* Is it okay to jaywalk with children across a busy street?
* Is it fair to give the bigger piece of chicken to the boy, rather than the girl, because "he's a boy"?
* Which is better-tasting, peaches or plums?
(I'm not arguing for moral relativism. There are some moral questions where everyone agrees on the answer.)