From Blue to Yellow and Green: US rivers are changing colors


  • For the past 36 years, American rivers have changed in colors, from blue to yellow and green — satellite images revealed.
  • Scientists explained that rivers can appear in different shades based on the amount of sediment, algae, pollution or dissolved organic matter.
  • Further, other factors, including man-made dams, reservoirs and agriculture development, can also cause river waters to change in color.

Satellite images showed that a third of American rivers have changed colors for the past 36 years.

Researchers analyzed 235,000 satellite photos, taken from 1984 to 2018, showing that a third of US rivers turned from blue to yellow and green. In total, they collected 16 million measurements on the 67,000 miles of rivers in the US.

In the North and West, river trend showed greener in color, while the eastern regions had yellow rivers. The Ohio basin and Upper Mississippi basin, which are larger waterways, moved to blue-green.

“Most of the rivers are changing gradually and not noticeable to the human eye,” lead author John Gardner, a postdoctoral researcher in the global hydrology lab at University of North Carolina, told Live Science. “But areas that are the fastest changing are more likely to be man-made.”  

The amount of suspended sediment, algae, pollution or dissolved organic matter in the water sets river shades, which can appear blue, green, yellow or other colors.

Generally, river water turns green when it carries less sediments, or if more algae blooms; and becomes yellow if more sediments stay in the water.

“Sediment and algae are both important, but too much or too little of either can be disruptive,” the hydrology lab lead researcher said.

River color can also be seasonal, just as leaves turn red and gold in fall. The color of this waterway varies due to many factors, such as rainfall and snowmelt.

The satellite images also showed hotspots influenced by man-made dams, reservoirs, agriculture and urban development, which may have had an impact on the color of some rivers — although not permanent.

To some extent, the satellite images helped scientists in assessing river health by measuring its colors, which are one of the elements to figure out what’s causing the change. “It is a very simple metric, which is integrating so many things,” Gardner said. “But it can be used to identify areas that are changing really fast.”

It is important for researchers to further determine the accuracy of river color in the assessment of ecosystem health.

These findings were published in the journal Geophysical Research Letters on December 6.

Source: Live Science

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