I was intrigued by the question Swimming in a river in England or Wales with a right of navigation because there was a tacit assumption that most rivers in England, Wales and Scotland -- or most parts of most rivers -- were what the U.S. Clean Water Act calls swimmable/fishable.
Obviously we can't have a multitude of questions asking if the portion of river R in country C from point A to point B is swimmable/fishable by USCWA standards, but questions about how water quality affects outdoor activities should be on-topic. We have several questions about giardia and drinking from streams in the wilderness, but that seems to be it.
I am just wandering here, and hope there are some more pointed thoughts on how to bring water quality -- and air quality -- into our forum.
As an example: Would a question about what percentage of river length (this would have to be qualified or defined in some way) in England was safe to swim in, and whether environmentally non-swimmable stretches were posted or otherwise indicated, be on topic?
And of course, immediately after I posted this Meta question, I saw this very focussed question on water quality: What are the most likely causes of unfilterable water contamination in upland UK areas?. Are good, focused questions like this the only way to go?