(Newser)
– A bird’s eye view of American rivers shows that about 1 in 3 have changed colors in the last three decades, reports Live Science. Specifically, they have gone from blue to shades of yellow or green since 1984, according to a comprehensive analysis of satellite images presented in a new study. Researchers do not present this as a report of doom and darkness, because so many variables are involved, but they say that their relatively simple metrics can be another tool in measuring waterway health. In general, rivers turn yellow due to excess sediment and green due to excess algae, while blue suggests “clean and healthy water systems”, on Smithsonian. According to the number of the new report, which analyzed more than 235,000 images from 1984 to 2018, more than half of US rivers were yellow, about a third were green and 8% were blue. See this interactive map.
“Most rivers change gradually and are not visible to the human eye,” says lead author John Gardner of the University of North Carolina for Live Science. “But the areas that change the fastest are more likely to be man-made.” Think of dams, reservoirs and increased development, all of which can change “nutrient flows” in rivers, a scientist from the University Corporation for Atmospheric Research told Salon. An atmospheric science professor at Penn State adds that climate change could also be a factor, as warmer temperatures would generally raise algae levels. However, “I don’t think there are immediate threats to human health” because of the changing color, says Gardner. While rivers can change colors seasonally due to factors such as melting snow, Figure 1-in-3 reflects rivers that have undergone long-term changes. (Read more river stories.)