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A new study finds that about one in three major U.S. rivers appear to be changing color since 1984, with many bodies of water slowly appearing to turn yellow and green – and scientists tell Salon that could mean very bad things for it. man. health.
Analyzing approximately 15.9 million satellite images taken over a period of more than three decades, researchers at the University of North Carolina at Chapel Hill, the University of Pittsburgh and Colorado State University found that Of more than 108,000 kilometers (about 67,000 miles) of rivers at least 60 meters (197 feet) wide surveyed across the country, 56% appeared predominantly yellow and 38% appeared predominantly green. While rivers often change color with seasons and flow regimes, scientists found that one-third of rivers experienced long-term “significant color changes” between 1984 and 2018. (If you will see what happened with your local river, there’s a handy interactive map here.)
“One thing to remember is that rivers don’t necessarily turn ‘yellow’ or ‘green’,” says Dr. John Gardner, co-author of the study (which was published earlier this month in the scientific journal Geophysical Research Letters), written to Salon. “If you remember the ROYGBIV visible light spectrum (red, orange, yellow, green, blue, purple), a significant ‘redshift’ trend just means that there is a trend towards the red yellow end of the spectrum, which could mean a river goes from green-blue to green. Likewise, a “blue-shifted” trend means that a river is heading towards the blue / green end of the spectrum, which could mean that a river turns from yellow-orange to yellow, or green-yellow to green. “
As Gardner pointed out, there are a number of things that can cause these noticeable color changes. When a river appears yellow, the likely culprit is suspended sediment; when it looks green, the likely cause is algae; and if a river is blue … well, like the cliché says, there’s a good chance that means you’re dealing with ‘relatively clear waters’.
The end result is that “if a river turned from green to yellow it would likely be due to a combination of less algae and more suspended sediment. The potential factors for these changes are many, covering changes in the channel. itself of how entire landscapes are managed. “Gardner observed that an increase in construction due to urbanization, increased river flow, and agriculture can increase suspended sediment and make a river more yellow, while a decrease in nutrients and light can reduce algae and make a river less green.
“A lot of the rivers that turn yellow quickly tend to be near the upstream ends of reservoirs,” wrote to Salon Dr. Tamlin Pavelsky, who also co-authored the article and works with Gardner at the Global Hydrology Lab. He cited as an example the upstream end of Lake Mead on the Colorado River, which he said changed color due to the deposition of sediment in reservoirs, “creating a ‘delta’ and causing the river to expand. further into the reservoir, we transform a relatively blue / green environment (a reservoir) into a relatively yellow environment (a river). “
He added: “It is a little less clear what causes some rivers to become greener. They may carry less sediment; there is evidence that this is the case in all American rivers, and we are working on a detailed analysis using satellite imagery. This may also be because they are experiencing increasing algal growth, or both. “
The two scientists cautioned against drawing definitive conclusions about whether these color changes pose risks to human health, noting that due to the many variables that can cause rivers to change color Whether a change in color indicates danger depends on why a river has changed its color.
“Interpreting color changes in terms of threats to human health can be tricky, because color incorporates so many different signals,” Pavelsky explained, listing dissolved organic matter alongside algae and sediment as possible causes of the color change of rivers. “In general, however, some rivers that become greener can experience increased growth of algae, which can sometimes be harmful to humans.” He also said that rivers that turn yellow “may experience an increase in the amount of sediment they carry, which may be a sign of greater soil erosion of upstream areas or other changes in the soil. the characteristics of the river caused by human activities “. like those linked to dams and reservoirs.
Gardner echoed these observations, telling Salon that “I don’t believe there are any immediate threats to human health (due to long-term changes in the color of the river).” Although nature has already put dissolved organic matter, sediment and algae in most rivers, “too much or not enough (depending on the river) can be harmful to the ecosystem and to human health”. He cited the Ohio River in 2015 as an example of a situation where algal blooms “can decrease the levels of oxygen that fish need to breathe and can also produce toxins.” In contrast, “many pollutants (such as mercury) move attached to suspended sediments, but generally in the large rivers included in our study there is too little suspended sediment, which is problematic for coastal wetlands. who need river sediment to keep up with sea level. “
He pointed out that “we only measured color, which is the combined effect of sediment, algae and dissolved organic matter on how water appears to the human eye. Therefore, we cannot say exactly what changes the color of the river and whether this is a problem for human (or ecosystem) health. “
Salon also contacted climatologists who said the results of the new study could, in some cases, be linked to human-caused global warming.
“There are a number of human impacts that lead to these,” Dr. Michael E. Mann, a distinguished professor of atmospheric sciences at Penn State University, told Salon. “Pollution from runoff in the form of agricultural fertilization can lead to algae blooms (usually cyanobacteria) that turn the water yellow, green or red. But climate change is also a factor. Warmer waters lead to anoxic conditions which promote algae blooms. So in short, the changes reported here are the consequence of multiple damage to the environment by human activities, including climate change.
Dr Kevin Trenberth, distinguished senior scientist in the climate analysis section at the National Center for Atmospheric Research of the University Corporation for Atmospheric Research, made a similar point to Salon, writing that “without doubt the dominant source of the changes more concerns people in more places are doing more things “like changing the way they use the land, building dams and making changes that interfere with the flow of nutrients in rivers. He said that “the effects of climate change are undoubtedly also at play, but are probably somewhat less important,” citing as examples changes in precipitation regimes, the speed with which snow melts and increasing snowfall. forest fires.
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