Interesting way of judging water quality, courtesy of Wired via Lifehacker. There are more scientific ways too in .
“Stoneflies (Order Plecoptera), Mayflies (Order Ephemeroptera) and Caddisfly (Order Tricoptera) are as suitable swimming pals as dolphins. Trout fishermen have been using these species as an indicator of good fishing for ages. These insects need healthy water to survive, so just by finding some of them in your recreational site, chances are high that the water quality is good.”
(Photo of Mayfly courtesy of Richard Bartz / Wikimedia Commons, CC BY-SA 2.5