A team of researchers with members from the University of North Carolina at Chapel Hill and Colorado State University reported that nearly a third of rivers in the United States have changed color over the past 34 years. In their paper published in the magazine Geophysical Research Letters, The group describes their study of thousands of satellite images taken from the United States and what they learned from it.

River health is important to a large number of people in the United States and many other countries, however, as researchers note in this new effort, there have been few tests of river water to monitor levels of pollution or sediments. To find out more about the health of American rivers in general, researchers took 235,000 and studied them Satellite images From the United States over the years 1984 to 2018. Compare Satellite images More than 34 years old for most major rivers at least 60 meters wide in the United States. In total, the team used 16 million measurements that encompassed 108,000 kilometers of the river.

Researchers found that nearly a third of all major rivers have changed color over the past 34 years. Half of them showed rivers that were mostly yellow – a sign that they were heavily loaded with sediment – and some of the others were mostly green – an indication of the presence of large amounts of algae. Only 8% of Satellites The photos showed rivers that were mostly blue.

River color is not necessarily an indicator of a river’s health, the researchers note. Increases in sediments could be due to increases in precipitation, for example. On the other hand, green rivers are often the result of fertilizer runoff from farms and meadows. Previous research has shown that fertilizers in US rivers make their way into the ocean, leading to algal blooms and dead spots as algae deplete the oxygen in the water.

The researchers note that most of the color changes they observed were gradual, with rivers changing color slowly over many years. They also noted that some rivers have changed to less natural colors, likely due to pollution or other dissolved organic matter. They also found that rivers in the west and north tended to turn from blue to green, while rivers in the east tended to turn yellow.

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