A third of America’s rivers have changed color significantly over the past 36 years, changing from blue to yellow and green, revealing new images.
Researchers analyzed 235,000 satellite images from the NASA and US Geological Survey (USGS) Landsat programs – a period of 34 years between 1984 and 2018. The changing theater can be seen in a Interactive map.
More than half of those satellite images showed rivers with a prominent place in yellow, while more than a third of the images were mostly green. Just 8% of the river’s photos were mostly blue.
“Most rivers are changing slowly and are not noticeable to the human eye,” said lead author John Gardner, a postdoctoral researcher at the Global Hydrology Lab at the University of North Carolina. “But areas that are changing the fastest are more likely to be man-made.”
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Depending on the amount of sediment, algae, pollution, or dissolved organic matter suspended in the water, rivers may appear in shades of blue, green, yellow, or other colors. As a general rule, the river water turns green as more algae, or when the water sediments less. The rivers turn yellow when they carry more sediment.
“Both silt and algae are important, but either very or very little can be disruptive,” Gardner said.
In total, the researchers collected 16 million measurements over a period of 34 years on rivers over 34,000 miles (108,000 kilometers) in the US that are more than 197 feet (60 meters) wide. This allowed them to track significant trends in color change over time.
More than half of the rivers, or 55%, varied in color over time, but no clear trend over time. A third color changed over that time period, and only 12% had a continuous color.
To the north and west, the rivers became greener, while the eastern areas of UShead tended to pale rivers. Large waterways, such as the Ohio Basin and Upper Mississippi Basin, also went blue-green.
“Large trends in yellow or green can be worrisome,” Gardner said.
Just as the leaves turn red and gold in the fall, rivers can also change color with the seasons, due to rain, snow, and other factors that change the flow of the river.
Satellite images revealed hotspots 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.
“You can see these trends going completely back in the other direction,” Gardner said, “especially if changes are occurring due to local mismanagement that is easily fixed.”
Monitoring the health of the river
Although the color of the river may not give an exact number for water quality or ecosystem health, it is a good proxy for both of these things. Color is also much easier for scientists to measure than water quality thanks to satellite imaging.
“It’s a very simple metric, integrating so many things,” Gardner said. “But it can be used to identify areas that are changing really fast.”
He said that scientists can find out what are the reasons for the change.
More research is needed to determine how true a river’s color is to determine ecosystem health.
The researchers’ findings in the journal were published on 6 December Geophysical research paper.
Originally published on Live Science.