Color coding rivers

America’s rivers are changing colors, moving from blue to shades of green and yellow, a group of researchers says.

A team of scientists pored over hundreds of thousands of river images dating back to 1984 and published their findings in the journal Geophysical Research Letters in December. They found about a third of American rivers have changed color, according to lead author John Gardner of the University of North Carolina at Chapel Hill.

People have noted the changing colors of rivers for centuries. But until now, scientists haven’t been able to use a river’s color to gauge its health. Blue rivers are typically clean rivers with healthy ecosystems. Excess sediment can change a river’s color to yellow. Green indicates algae.

“It is a very simple metric, which is integrating so many things,” Gardner, a postdoctoral hydrology researcher, told Live Science. “It can be used to identify areas that are changing really fast.”

Researchers scrutinized nearly 235,000 satellite images from 1984 to 2018 covering 67,000 miles of rivers from NASA and the U.S. Geological Survey. They restricted the study to large rivers wider than 197 feet.

Many rivers change colors seasonally due to rainfall or snowmelt, but a full third in the study underwent long-term shifts. In the original photographs, America’s rivers appeared bluer than today. More than 20 percent of the nation’s rivers shifted toward green, especially in western states. Scientists found a trend away from blue and toward yellow in the northeast. According to the report, yellow rivers are now predominant, accounting for 56 percent of the waters they studied. Another 38 percent appeared green while the rest appeared blue. After compiling the data, the researchers created an interactive map to demonstrate their findings. Study co-author Tamlin Pavelsky told Salon the yellow shifts are correlated to the presence of reservoirs on the river. What’s causing green shifts is less clear.

Both Pavelsky and Gardner cautioned against reading too much into longer term color shifts. “Most of the rivers are changing gradually and not noticeable to the human eye,” Gardner told Live Science. “But areas that are the fastest changing are more likely to be man-made.”