A new study shows that about one in three major U.S. rivers has changed color since 1984, and many of the water bodies appear to be slowly turning yellow and green – and scientists tell Salon that it can do many bad things to humans. mean. health.
Researchers at the University of North Carolina at Chapel Hill, the University of Pittsburgh and Colorado State University analyzed about 15.9 million satellite images taken over a period of more than three decades and discovered that of the more than 108,000 kilometers (about 67,000 miles) rivers. studied at least 60 meters (197 feet) wide across the country, 56% appeared predominantly yellow and 38% predominantly green. While rivers often change color based on the seasons and flow regimes, scientists found that a third of the rivers experienced long-term “significant color shifts” between 1984 and 2018. (If you want to see what happened to your local river ?, here’s a handy interactive map.)
“One thing to remember is that rivers do not necessarily become ‘yellow’ or ‘green’,” said Dr. John Gardner, co-author of the study (published earlier this month in the scientific journal Geophysical Research Letters). written to Salon. “If you remember the visible light spectrum ROYGBIV (red, orange, yellow, green, blue, violet), a significant ‘red shifted’ trend simply means that there is a tendency towards the red yellow point of the spectrum, which may means a river changes from green-blue to green A ‘blue-hidden’ trend also means that a river tends to the blue / green end of the spectrum, which can mean a river from yellow-orange to yellow changed, or green-yellow to green. “
As Gardner pointed out, there are a number of things that can cause these apparent color shifts. If a river looks yellow, the probable culprit is hanging sediment; if it appears to be green, the probable cause is algae; and if a river is blue … well, as the cliché says, chances are you’re dealing with ‘relatively clear waters’.
The end result is that “if a river changes from green to yellow, it will probably be caused by a combination of less algae and more erect sediment. Potential drivers for such changes are numerous, exceeding changes to the river channel itself to how the entire landscapes are managed. “Gardner noted that increased construction due to urbanization, increasing river flow and agriculture could increase the hanging sediment and make a river look more yellow, while a decrease in nutrients and light algae could reduce and a river can make it look less green.
“Many of the rivers that turn yellow quickly are usually near the upstream ends of the reservoirs,” said Dr. Tamlin Pavelsky, who also co-wrote the article and works with Gardner in the Global Hydrology Lab, wrote to Salon. He cites as an example the upstream end of Lake Mead on the Colorado River, which according to him changed color because sediment was deposited in reservoirs, built up a ‘delta’ and caused 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: “What causes some rivers to be greener is a little less clear. It could be that they carry less sediment; there is evidence that this is the case in U.S. rivers in general, and we are working on a detailed analysis using satellite images. It could also be because they are experiencing algae growth or both. “
Both scientists do not warn against drawing definitive conclusions about whether these color changes affect human health. Because of the many variables that can cause rivers to change color, the question of whether a color change poses a danger depends on the specific reasons why a particular river has changed its hue.
“The interpretation of color changes in terms of threats to human health can be difficult because color integrates so many different signals,” Pavelsky explained, listing the dissolved organic matter along with algae and sediment as possible causes of the change of rivers . “In general, however, some rivers that become greener experience algae growth, which can sometimes be harmful to humans.” He also said that rivers that turn yellow may have an increase in the amount of sediment they transport, which may indicate more erosion of soil from upstream areas or other changes in the characteristics of the river due to human activities. ‘such as those connected to dams and reservoirs.
Gardner agrees with these observations and tells Salon that “I do not believe there are any immediate threats to human health (due to long-term changes in river color).” Although nature has already placed dissolved organic matter, sediment and algae in 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 a situation in which algae blossoms can “reduce oxygen concentrations that fish must inhale and also produce toxins.” In contrast, ‘many pollutants (such as mercury) move to suspended sediment, but in the large rivers included in our study, there is too little suspended sediment which is problematic for coastal wetlands that need sediment to keep up with the sea level. rise. ‘
He emphasized that ‘we only measured color, which is the combined effect that sediment, algae and dissolved organic matter have on how water occurs in the human eye. Therefore, we can not say exactly what changes the color of the river and whether it is problematic for human (or ecosystem health). “
Salon also reached out to climate scientists who said the findings of the new study could, in some cases, be linked to man-made global warming.
“There are a number of human consequences that lead to this,” said Dr. Michael E. Mann, a leading professor of atmospheric science at Penn State University, told Salon by email. “Runoff pollution in the form of agricultural fertilizers can lead to algae blooms (typically cyanobacteria) that make the water yellow, green or red. But climate change is also a factor. Warmer waters lead to anoxic conditions that are favorable for algae flowers. In short “the changes reported here are the result of multiple environmental insults through human activities, including climate change.”
Dr. Kevin Trenberth, a leading senior scientist in the division of climate analysis at the National Center for Atmospheric Research at the University Corporation for Atmospheric Research, made a similar point to Salon and wrote that ‘undoubtedly the dominant source of the changes concerns more people in more places doing more things, such as using land, building dams, and altering nutrient flow in rivers. He said that ‘the effects of climate change are undoubtedly also at stake, but probably a little less’, citing as examples changes in rainfall patterns, the rapid amount of snow and the increase in veld fires.