NASA satellite images show blue US rivers turning yellow and green, here’s why

13:27 GMT 01.01.2021Get a short url

Although the study detected areas where human activity can affect the color of some rivers, these changes are not necessarily permanent.

After examining some 235,000 satellite images taken between 1984 and 2018 via Landsat, a joint program between NASA and the US Geological Survey, a team of scientists determined that a third of rivers in the United States exhibited a color change significant over the past three and a half decades, reports Live Science.

According to the media outlet, more than half of the images in question showed rivers “with a dominant shade of yellow”, a third of the images “were predominantly green” and only 8% of the river images were “predominantly blue”.

“Most rivers change gradually and are not visible to the naked eye,” said John Gardner, postdoctoral researcher at the University of North Carolina’s Global Hydrology Laboratory and lead author of the study. “But the areas that change the fastest are more likely to be human-made.”

During their study, the research team collected “16 million measurements” over a 34-year period of approximately 108,000 kilometers of rivers over 60 meters wide in the United States.

Since the coloring of a river is generally determined by “the amount of suspended sediment, algae, pollution or organic matter dissolved in the water”, the media notes that the water in the river tends to turn green. “as more algae blooms, or as the water carries less sediment”, while the yellow color probably means the river is carrying more sediment.

“The major trends in yellow or green can be worrisome,” Gardner noted, adding that “it depends on the individual river.”

The study also found that about 55 percent of these rivers varied in color but did not show a clear trend, and one-third of the rivers changed color; the color of 12 percent of the rivers in question remained constant.

And while satellite images have shown areas where human activity, such as “dams, reservoirs, agriculture and urban development”, can affect the color of some rivers, such changes are not necessarily permanent, adds The media.

“You can totally see these trends going the other way,” Gardner explained, “especially if the change happens because of local mismanagement that is easily corrected”.