new a study It reveals that nearly one in three major American rivers have changed color since 1984, as many bodies of water seem to be slowly turning yellow-green – and scientists tell Salon this could mean some very bad things to human health.

Analyzing nearly 15.9 million satellite images captured over more than three decades, researchers at the University of North Carolina at Chapel Hill, the University of Pittsburgh and Colorado State University discovered that of the more than 108,000 kilometers (about 67,000 miles) of rivers, 60 meters were studied. (197 ft) at least across the country, 56% appearing mostly yellow and 38% appearing mostly green. While rivers often change color based on seasons and flow regimes, scientists have found that a third of rivers experienced „major changes in color“ in the long term between 1984 and 2018. (If you want to know what happened to your local river, there is a handy one. An interactive map is here.)

„One thing to remember is that rivers don’t necessarily turn“ yellow „or“ green, „said Dr. John Gardner, co-author of the study (published earlier this month in the scientific journal Geophysical Research Letters), wrote for Salon If you remember the ROYGBIV visible light spectrum (red, orange, yellow, green, blue, and purple), the large ‚red‘ direction simply means that there is a trend towards the red yellow end of the spectrum, which could mean the river changes from green and blue to green. Likewise, a “blue” direction means that the river is heading towards the blue / green end of the spectrum, which could mean that the river changes from yellow-orange to yellow, or green-yellow to green. „

As indicated by Gardner, there are a number of things that could cause these apparent color changes. When the river appears yellow, the likely culprit is suspended sediment; When it appears green, the most likely cause is algae; And if the river is blue … well, as it is in the cliché, odds are you are dealing with „relatively clear water.“

The end result is that “if the river changes from green to yellow, it is likely to be caused by a mixture of less algae and more suspended sediments. The potential drivers of such changes are numerous, and changes in the river channel itself extend to how the entire landscape is managed.” „ Gardner noted that increased construction due to urbanization and increased river flows and agriculture can increase suspended sediments and make the river appear more yellow, while lower nutrients and light can reduce algae and make the river appear less green.

Dr. Tamlin Pavlsky, who also co-authored the paper and works with Gardner at the Global Hydrology Laboratory, wrote to Salon “Lots of rapidly turning yellow rivers tend to be near the upstream ends of reservoirs.” He cited an example of the upstream end of Lake Mead on the Colorado River, which he said had changed color due to sediment deposition in reservoirs, “building a delta” and making the river extend into the reservoir. When this happens we convert a relatively blue / green environment (reservoir) to a yellow environment. Relatively (River) „.

He added: „What makes some rivers greener is less clear. The reason may be that they transport fewer sediments; there is evidence that this is the case in American rivers in general, and we are working on detailed analysis using satellite images. That could be.“ Also because they are experiencing an increased growth of algae or both. „

Both scientists cautioned against drawing final 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, the question of whether a color change indicates a risk depends on the specifics of why there is The river has changed its color.

“Interpreting color changes in terms of risks to human health may be difficult, because color incorporates many different signals,” Pavlsky said, citing dissolved organic matter along with algae and sediments as possible causes of river color change. „In general, though, some rivers that are greener may see increased growth of algae, which can sometimes be harmful to humans.“ He also said that rivers that turn yellow „may see increases in the amount of sediments they transport, which could be an indication of further soil erosion from upstream areas or other changes in river characteristics due to human activities“ such as those associated with dams and reservoirs.

Gardner echoed these observations, telling Saloon, „I don’t think there are any immediate threats to human health (due to long-term changes in the river’s color).“ Although nature has already put dissolved organic matter, sediments and algae into most rivers, „Too much or too little (depending on the river) can be harmful to the ecosystem and human health.“ He cited the Ohio River in 2015 as an example of an algal bloom that „can reduce the oxygen concentrations that fish need to breathe and can also produce toxins.“ In contrast, “many pollutants (such as mercury) move associated with suspended sediments, but in general in the large rivers included in our study there is very little suspended sediment that is a problem for coastal wetlands that need river sediments to keep up with sea level” Go up. „

„We only measured color, which is the combined effect of sediments, algae and dissolved organic matter on how water appears to the human eye. Therefore, we cannot determine what changes the color of the river and whether it is a problem for humans (or the health of the ecosystem),“ he stressed.

Salon has also reached out to climate scientists who said the new study results could, in certain cases, be linked to anthropogenic global warming.

„There are a number of human influences that lead to this,“ Dr. Michael E. Mann, Distinguished Professor of Atmospheric Sciences at Pennsylvania State University, told the salon via email. “Runoff pollution in the form of agricultural fertilization can lead to algal blooms (usually cyanobacteria) that turn the water yellow, green, or red. But climate change is a factor as well. Warm waters lead to hypoxic conditions that favor algal blooms. So in short, the changes reported here are the result of multiple environmental insults by human activities, including climate change. ”

Dr. Kevin Trenberth, a distinguished distinguished scientist in the Department of Climate Analysis at the National Center for Atmospheric Research at the University Foundation for Atmospheric Research, made a similar point to Saloon, writing that “There is no doubt that the dominant source of change is about more. People in more places do more. Things „like changing how they use the land, building dams and changes that interfere with nutrient flow into rivers.“ “The impacts of climate change undoubtedly play a role as well, but they are likely to be somewhat smaller,” he said, citing for example changes in precipitation patterns, the speed at which snow melts, and an increase in forest fires.