One-third of rivers in the United States turn from blue to green and yellow

A team of researchers made up of members of the University of North Carolina at Chapel Hill and Colorado State University report that about a third of rivers in the United States have changed color in the past 34 years. In their article published in the journal Geophysical research letters, the group describes their study of thousands of satellite images taken of the United States and what they learned from them.

The health of rivers is important to many people in the United States and many other countries, but, as researchers note in this new effort, little testing has been done on river water to monitor the pollution or sediment levels. To learn more about the health of American rivers in general, the researchers obtained and studied 235,000 satellite images of the United States taken during the years 1984 to 2018. They compared satellite images over 34 years for most of the major rivers at least 60 meters wide in the United States In total, the team used 16 million measurements covering 108,000 kilometers of river.

Researchers have found that about a third of all major rivers have changed color in the past 34 years. Half of them showed mostly yellow rivers – a sign that they were heavily loaded with sediment – others were mostly green – an indication of large amounts of algae. Only 8% of the satellite images showed mainly blue rivers.

The color of the river is not necessarily an indicator of the health of the river, as the researchers point out. The increase in sediment could be due to increased precipitation, for example. Green rivers, on the other hand, are very often the result of fertilizer runoff from farms and lawns. Previous research has shown that fertilizers in American rivers head to the ocean, resulting in algae blooms and dead spots as the algae deplete oxygen in the water.

The researchers note that most of the color changes they observed were gradual, as rivers slowly changed color over many years. They also noted that some rivers had changed to less natural colors, possibly due to pollution or other dissolved organic matter. They also found that the western and northern rivers tended to turn blue to green, while the eastern rivers tended to yellow.

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