A third of the U.S. rivers have changed significantly in color over the past 36 years, from blue to yellow and green, and striking new images.
Researchers analyzed 235,000 satellite images – taken over a 34-year period between 1984 and 2018 – from NASA and the U.S. Geological Survey (USGS) Landsat program. The changing colors can be in a interactive map.
More than half of the satellite images have rivers with a dominant tint of yellow, while more than a third of the images were mostly green. Only 8% of the river photos were mostly blue.
“Most of the rivers are gradually changing and are not noticeable to the human eye,” lead author John Gardner, a postdoctoral researcher in the Global Hydrology Laboratory at the University of North Carolina, told WordsSideKick. “But areas that change the fastest are more likely to be man-made.”
Related: Rainbow rivers: see beautiful maps of the world’s waterways
Rivers can look like shades of blue, green, yellow or other colors, depending on the amount of suspended sediment, algae, pollution or dissolved organic matter in the water. Generally, river water turns green as algae blooms, or as the water carries fewer sediments. Rivers tend to turn yellow if they carry more sediment.
“Sediment and algae are both important, but too much or too little of both can be disruptive,” Gardner said.
Evaluation of trends
In total, the researchers collected 16 million measurements over the 34-year period on the 108,000 kilometers of rivers in the US that are more than 60 meters wide. This enabled them to follow significant trends in color change over time.
More than half of the rivers, or 55%, differed over time, but over time no clear trend. A third changed color in that period, and only 12% had a constant color.
In the north and west, rivers mostly became greener, while the eastern regions of the US tended to turn yellow rivers. Larger waterways, such as the Ohio Basin and the Upper Mississippi Basin, also moved to blue-green.
“Major trends toward yellow or green can be worrying,” Gardner said, adding that “it depends on the individual river.”
Just as leaves turn red and gold in the fall, rivers can also change color with the seasons, thanks to changes in rainfall, snowmelt and other factors that alter the flow of a river.
The satellite images did reveal hotspots where human influences, such as dams, reservoirs, agriculture and urban development, could change the color of some rivers.
But these changes are not necessarily permanent.
“You could see these trends reversing completely in the other direction,” Gardner said, “especially if the change is due to local mismanagement that can be easily corrected.”
Monitoring the health of the river
Although river color may not give exact figures for water quality or ecosystem health, it is a good approach for both of these things. Thanks to satellite imagery, it is also much easier for scientists to measure color than water quality.
“It’s a very simple measure that integrates so many things,” Gardner said. “But it can be used to identify areas that are changing really fast.”
From there, scientists were able to figure out what was causing the change, he said.
More research is now needed to determine how accurate the river color is to determine the health of the ecosystem and what changes are important to monitor.
The researchers’ findings were published in the journal on December 6. Geophysical research letters.
Originally published on Live Science.