A bird's-eye view of American rivers shows that roughly 1 in 3 have changed colors over the last three decades, reports Live Science. Specifically, they've gone from blue to shades of yellow or green since 1984, according to a comprehensive analysis of satellite images laid out in a new study. Researchers aren't presenting this as a doom-and-gloom report because so many variables are involved, but they say their relatively simple metric can be one more tool in gauging the health of waterways. Generally, rivers turn yellow because of excess sediment and green because of excess algae, while blue suggests "clean, healthy water systems," per Smithsonian. By the count 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 of the rivers are changing gradually and not noticeable to the human eye," lead author John Gardner of the University of North Carolina tells Live Science. "But areas that are the fastest changing are more likely to be man-made." Think dams, reservoirs, and increased development, all of which can change "nutrient flows" into the rivers, a scientist at the University Corporation for Atmospheric Research tells Salon. A professor of atmospheric science at Penn State adds that climate change also might be a factor because warmer temperatures would generally raise algae levels. Still, "I do not believe there are any immediate threats to human health" because of the shifting colors, says Gardner. While rivers can change colors seasonally because of factors such as snow melt, the 1-in-3 figure reflects rivers that have undergone long-term shifts. (Read more river stories.)