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A third of the rivers in the United States have changed color dramatically in the last 36 years, turning from blue to yellow and green, according to stunning new photos.
The researchers analyzed 235,000 satellite images, taken over a 34-year period between 1984 and 2018, by NASA, the United States Geological Survey (USGS) and the Landsat program. The changing shapes can be viewed on an interactive map.
More than half of the satellite images showed rivers with a predominant yellowish hue, while more than a third of the images were mostly green. Only 8% of the river images were blue.
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Lead author John Gardner, a postdoctoral researcher at the Global Hydrology Laboratory at the University of North Carolina, told Live Science: “Most rivers change gradually and are not perceptible to the human eye. But the regions that change as quickly as possible they can be created by humans. “
Rivers can appear as shades of blue, green, yellow, or other colors, depending on the amount of suspended sediment, algae, pollution, or dissolved organic matter in the water.
As a general rule of thumb, river water turns green as more algae grows or when the water contains less sediment. Rivers tend to turn yellow when they carry more sediment.
“Sediment and algae are important, but too much or too little of any of them can be harmful,” Gardner said.
In all, the researchers collected 16 million measurements over a 34-year period across 108 km of rivers in the United States that were more than 60 meters wide. This allowed them to track important trends in color change over time.
More than half of the rivers, or 55%, have varied over time, but without a clear trend over time. A third color change in that time period, and only 12% had a stable color.
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In the north and west, rivers tended to become greener, while the eastern regions of the United States tended toward yellow rivers. Large waterways, such as the Ohio Basin and the Upper Mississippi Basin, also turned blue and green.
“Big trends toward yellow or green can be unsettling,” Gardner said, but added that “it depends on the river individually.”
Just as leaves turn red and gold in autumn, rivers can also change color with the seasons, thanks to changes in rainfall, snowmelt, and other factors that alter river flow.
Satellite images revealed hot spots where human influences, such as dams, reservoirs, agriculture, and urban development, can change the color of some rivers. But these changes are not necessarily permanent.
Although the color of the river cannot give precise figures on the quality of the water or the health of the ecosystem, it is a good alternative to both. Thanks to satellite images, it is easier for scientists to measure color than water quality.
“It’s a very simple scale that integrates a lot of things. But it can be used to identify areas that change very quickly,” Gardner said. From there, scientists can figure out what caused the change.
More research is needed now to determine how accurately the coloration of the river determines the health of the ecosystem and what important changes to observe.
Source: Live Science
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