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America’s rivers are changing color, and people are behind many of the changes, according to a new study.

One third of the tens of thousands of two kilometers long river segments in the United States they have markedly changed color in satellite images since 1984.
That includes 18,715 kilometers that turned greener or headed toward the violet end of the color spectrum, according to a study published in this week’s journal. Geographic Research Letters. Some river segments turned redder.

Only about five percent of the mileage on American rivers is considered blue, a color the general public often equates with crystal clear waters.

About two-thirds of America’s rivers are yellow, indicating they have a lot of land.

But 28 percent of rivers are green, often indicating that they are saturated with algae. And the researchers found that two percent of America’s rivers over the years went from a predominant yellow to a distinctive green.

“If things are getting greener, that’s a problem,” said lead study author John Gardner, a professor of geology and environmental sciences at the University of Pittsburgh.

Although a green tint in rivers can be normal, Professor Gardener said it often means large blooms of algae that cause oxygen loss and can produce toxins.

The main causes of the color changes are runoff from agricultural fertilizers, dams, efforts to combat soil erosion and man-made climate change, which increases water temperature and rain-related runoff, they said. the study authors.

“We change our rivers a lot. A lot of that has to do with human activity, ”said study co-author Tamlin Pavelsky, a professor of global hydrology at the University of North Carolina.

For example, said Professor Pavelsky, the color green sometimes in the Ohio River indicates a serious algae problem due to agricultural runoff, while rivers that turn less yellow demonstrate the success of regulations in preventing soil erosion. .

The study analyzed more than 230,000 POT Satellite images over 35 years old, centered on rivers and reservoirs.

The study found that much of the shift to greener rivers occurred in the north and west, while yellowing occurred more in the east and around the Mississippi River.

He also found that some rivers change color naturally with the seasons.

Outside experts praised the study, saying that while difficult-to-understand measurements have shown problems with American rivers, this illustrates the situation simply.

The study “is great and a little mind-boggling (but intuitive),” Martin Doyle, director of water programs at Duke University, wrote in an email.

“It shows how most aspects of our planet are being affected by humans, now including the basic color of our water. That’s pretty profound if you think about it. “

“It is also important because it opens up the idea and the potential of using river color as an early stage indicator or warning of environmental change.”

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