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While the study has discovered areas where human activity can affect the color of some rivers, these changes may not necessarily be permanent.
After examining about 235,000 satellite images taken between 1984 and 2018 through Landsat, a joint program of NASA and the US Geological Survey, a team of scientists determined that one-third of the rivers in the United States have shown significant color change. for the past three and a half decades, Live Science reports.
According to the media, more than half of the images in question showed rivers “with a dominant shade of yellow,” a third of the images “were mostly green,” and only 8 percent of the river images were “mostly blue.” “.
“Most rivers change gradually and are imperceptible to the human eye,” said John Gardner, a postdoctoral researcher in the global hydrology lab at the University of North Carolina and lead author of the study. “But areas that change the fastest are more likely to be human-made.”
In the course of their study, the research team collected “16 million measurements” over 34 years on about 108,000 kilometers of rivers over 60 meters wide in the United States.
Because a river’s color is usually determined by the “ amount of suspended sediment, algae, pollution, or dissolved organic matter in the water, ” the media outlet notes that river water tends to turn green “ if more algae bloom or when the water carries less sediment ”, while the color yellow probably means that the river carries more sediment.
“Major trends towards yellow or green can be troubling,” Gardner noted, adding that “it depends on the individual river.”
The study also found that about 55 percent of these rivers varied in color, but showed no apparent trend, and one third of the rivers changed color; the color of 12 percent of the rivers involved remained constant.
And while satellite images showed areas where human activities, such as “dams, reservoirs, agriculture and urban development,” can affect the color of some rivers, such changes may not necessarily be permanent, the media adds.
“You could see these trends go completely in the other direction,” explained Gardner, “especially if the change comes from local mismanagement that can be easily resolved.”