A third of the rivers in the United States have changed color dramatically over the past 36 years, turning from blue to yellow and green, according to stunning new photos.
The researchers analyzed 235,000 satellite images, taken over a period of 34 years between 1984 and 2018, by NASA, the US Geological Survey (USGS) and the Landsat program. The changing shapes can be seen 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 noticeable to the human eye. But regions that change as quickly as possible may be human-made.”
Rivers can appear as shades of blue, green, yellow, or other colors depending on the amount of suspended sediments, algae, pollution, or organic matter dissolved in the water.
As a rule, river water turns green as more algae flourish, or when the water holds less sediment. Rivers tend to turn yellow when they carry more sediment.
“Sediments and algae are both important, but too much or too little of either of them can be disruptive,” Gardner said.
In total, the researchers collected 16 million measurements over a 34-year period on 108 km of rivers in the United States that were over 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 with no 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, the rivers tended to become greener, while the eastern regions of the United States had a trend towards yellow rivers. Large waterways, such as the Ohio Basin and the Upper Mississippi Basin, have also moved blue and green.
“The big trends to yellow or green may be disconcerting,” said Gardner, 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 precipitation, snowmelt and other factors that alter river flow.
Satellite imagery revealed hot spots where human influences, such as dams, reservoirs, agriculture and urban development, may change the color of some rivers. But these changes are not necessarily permanent.
Although the color of the river cannot give accurate figures for the quality of the water or the health of the ecosystem, it is a good alternative to both of these things. Thanks to satellite imagery, it is easier for scientists to measure color than water quality.
“It’s a very simple metric that integrates many things. But it can be used to identify areas that change very quickly,” Gardner said. From there, scientists can find out what caused the change.
More research is needed now to determine how accurately the river’s coloration is in determining the health of the ecosystem and which important changes to watch.
Source: Live Science
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