Yellowstone National Park's Morning Glory thermal spring used to be deep blue but its currentyellow and green hues were caused in part by tourists throwing coins, rocks, and assorted crap into the pool. The detritus has the effect of"partially blocking the underground heat source and lowering the temperature of the spring to a range habitable by photosynthetic microorganismsthat probably didn't live there before" and produce pigments that result in the yellow and green color, according to a Science Friday article.The image at right is from 1966

To read the complete article, see:
People throwing pennies changed the color of a Yellowstone hotspring (http://boingboing.net/2015/02/06/people-throwing-pennies-change.html)

To read the original Science Friday article, see:
Picture of the Week: Yellowstone's MorningGlory (http://sciencefriday.com/blogs/02/04/2015/picture-of-the-week-yellowstone-s-morning-glory.html)