Satellite images of rivers in the US have revealed a disturbing trend of color change: in the past three decades, one third of previously blue rivers have turned yellow or green. The discovery comes after an analysis of nearly 235,000 images taken between 1984 and 2018. Of the rivers that changed color, 56 percent were mostly yellow and 38 percent were mostly green.
The findings, published in the journal Geophysical Research Letters, come from a team of researchers who wanted to try to use the color of water to understand the health of a river. It may sound like a simple technique that even non-scientists could do, but the scientific community has seemingly avoided sightings of color in favor of obtaining real river samples.
Which would be fine if so few samples were not obtained.
“Rivers are among the most endangered ecosystems worldwide,” the authors wrote, “but we do not have a wide-scale understanding of their changing ecology, as most are rarely sampled.”
So the team thought broad, visual observations of river colors could help figure out the health of rivers with little data. Tracking the color can also give researchers a better idea of current trends, patterns and areas of alarm. And nowadays the quality of satellite images is so good that it could make river monitoring a lot easier.
“[River color] is a very simple statistic, “lead author John Gardner told Live Science.” But it can be used to identify areas that are changing very quickly. “
The research team also plotted the river colors on an interactive map, which provides a good view of the changes in the sky.
But what do the colors of the river mean? Well … scientists aren’t quite sure yet. Sometimes rivers change color due to sediment flowing down during seasonal snow melts. Sometimes rivers have a period in which a forest of algae turns it green temporarily. Other times, human activity causes environmental disturbance, which changes water quality and thus color.
It really depends on what is normal for the river which is why observing their color can be so important. It’s not super abnormal for rivers to be blue, green, yellow, or even brown. But if a sudden change occurs, it could be an indication that something is going wrong.
“Most rivers change gradually and are imperceptible to the human eye,” Gardner explained to Live Science. “But areas that change the fastest are more likely to be human-made.”
While more research is needed to know exactly what the color changes mean, the authors argued that tracking “water color can locate rivers undergoing rapid environmental changes and work towards a better understanding of rivers on a continental scale.”