Over the last 36 years, a third of the United States' rivers have changed color, shifting from shades of blue to yellow and green. The change in hue raises a red flag for the health of waterways nationwide, reports Harry Baker for Live Science.
Historically, people used a river's color to gauge its health, but modern-day scientists haven't really considered it as a measurement. A team of scientists led by John Gardner, an environmental scientist at the University of North Carolina at Chapel Hill, seized the missed opportunity. He and his team analyzed nearly 235,000 satellite images taken by NASA and the U.S. Geological Survey from 1984 to 2018 to see how river colors have changed over the decades, reports Peter Dockrill for Science Alert. They published their findings last month in the journal Geophysical Research Letters.
The team analyzed 67,000 miles of large rivers via satellite images and found that 56 percent appeared yellow and 38 percent green. Although rivers can change color throughout the year, such as from melted snow or changes in rainfall, the researchers found that about one-third of the rivers underwent long-term color shifts, reports Matthew Rozsa for Salon. They documented their findings in an interactive map.
Changes in color can clue scientists into what is happening in the water. Blue indicates clean, healthy water systems, but as a general rule, sediment causes rivers to turn yellow, and algae blooms lead to green hues. The researchers found that the more dramatic changes in color were clustered around dams, agriculture and urban areas, reports Live Science.
"Sediment and algae are both important, but too much or too little of either can be disruptive," Gardner tells Live Science. "Big trends to yellow or green can be worrying."
Since the team didn't collect water samples to correlate with the satellite images, they cannot definitively say what is driving these changes. However, they say that remotely studying rivers at a grand scale like this can help them monitor the health of waterways without needing to be in the field, reports Science Alert.
"It is a very simple metric, which is integrating so many [variables]," Gardner tells Live Science. "But it can be used to identify areas that are changing really fast."