A third of the rivers in the United States have transformed from blue to yellow and green over the last three decades, according to a new study from UNC Chapel Hill. While the cause is not entirely clear, the experts believe that the colors represent the presence of pollution, sediment, and algae.
Led by environmental scientist John Gardner, the team analyzed 235,000 satellite images captured from 1984 to 2018 by the NASA and USGS Landsat program. More than half of the rivers examined for the study had turned yellow, 38 percent had turned green, and only six percent were blue.
“Over the full record, 33 percent of rivers had significant trends in color, but the direction of the trend varied regionally,” wrote the researchers, noting that water color is also affected by the seasons. For example, the river colors shifted towards the red end of the spectrum during spring and summer.
Only 12 percent of the colors remained consistent, and most of these rivers were located in the central U. S. and lower Mississippi River. The researchers also noticed a trend in which rivers across the North and the West tended to be greener, while those in the East were predominantly yellow. Larger bodies of water, such as the Ohio basin, widely transitioned from blue to green.
Gardner said that while large trends toward green or yellow can be concerning, it depends on the individual river. In many cases, the satellite images could be used to pinpoint “hotspots” of human disturbances, where activities such as agriculture and urban development impacted the water.
Remotely observing the color of rivers may become a useful tool for monitoring the health of waterways worldwide. While this method does not directly use water samples, it can be extremely helpful for analyzing water on a large scale.
“Water color is directly measured by satellites, unlike specific water quality constituents, allowing rapid monitoring of water quality in the absence of in-situ observations,” wrote the researchers.
“The intuitive and easily observable nature of water color could enable collection of massive volumes of data across spatial and temporal scales for water quality monitoring, identifying global hotspots of change, and advancing macrosystems ecology in rivers.”
The findings are reported in Geophysical Research Letters.
By Chrissy Sexton, Earth.com Staff Writer