A new study reveals that roughly one out of three large American rivers have appeared to change color since 1984, with many of the bodies of water seeming to slowly turn yellow and green — and scientists tell Salon that this could mean some very bad things for human 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 discovered that of the more than 108,000 kilometers (roughly 67,000 miles) of rivers at least 60 meters (197 feet) wide studied throughout the country, 56% appeared predominantly yellow and 38% appeared predominantly green. While rivers often change color based on the seasons and flow regimes, the scientists found that one-third of rivers had experienced long-term "significant color shifts" between 1984 and 2018. (If you want to see what has happened with your local river, there is a handy interactive map here.)
"One thing to remember is that rivers are not necessarily turning 'yellow' or turning 'green,'" Dr. John Gardner, a co-author of the study (which was published earlier this month in the scientific journal Geophysical Research Letters), wrote to Salon. "If you recall to visible light spectrum R-O-Y-G-B-I-V (red, orange, yellow, green, blue, violet), a significant 'red-shifted' trend simply means that there is a trend towards the red yellow end of the spectrum, which could mean a river is changing from green-blue to green. Similarly, a 'blue-shifted' trend means a river is trending towards the blue/green end of the spectrum, which could mean a river is changing from yellow-orange to yellow, or green-yellow to green."
As Gardner pointed out, there are a number of things that can cause these seeming color shifts. When a river appears yellow, the likely culprit is suspended sediment; when it appears to green, the probable cause is algae; and if a river is blue... well, as the cliché goes, the chances are that means you're dealing with "relatively clear waters."
The end result is that "if a river changed from green to yellow, that would likely be caused by some combination of less algae and more suspended sediment. Potential drivers of such changes are numerous, spanning changes to the river channel itself to how entire landscapes are managed." Gardner observed that increased construction due to urbanization, increasing river flows and agriculture can increase suspended sediment and make a river appear more yellow, while a decrease in nutrients and light can reduce algae and make a river appear less green.
"A lot of the rivers that are rapidly turning yellow tend to be near the upstream ends of reservoirs," Dr. Tamlin Pavelsky, who also co-authored the paper and works with Gardner in the Global Hydrology Lab, wrote to Salon. He cited as one example the upstream end of Lake Mead on the Colorado River, which he said has changed color because of sediment being deposited into reservoirs, "building up a 'delta' and making the river extend further into the reservoir. When this happens we transform a relatively blue/green environment (a reservoir) into a relatively yellow environment (a river)."
He added, "What's causing some rivers to become greener is a little less clear. It could be that they are transporting less sediment; there's evidence that this is the case in U.S. rivers overall, and we're working on a detailed analysis using satellite imagery. It could also be because they're experiencing increasing growth of algae, or both."
Both scientists cautioned against drawing definitive conclusions about whether these color changes present risks to human health, noting that because of the numerous variables which can cause rivers to change color, the question of whether a color change indicates danger depends on the specifics of why a specific river has altered its hue.
"Interpreting color changes in terms of human health threats can be tricky, because color integrates so many different signals," Pavelsky explained, listing dissolved organic matter alongside algae and sediment as possible causes of rivers changing color. "In general, though, some rivers that are becoming greener may be experiencing increased algal growth, which can sometimes be harmful to humans." He also said that rivers which are turning yellow "may be experiencing increases in how much sediment they're transporting, which can be an indication of more erosion of soil from upstream areas or other changes in the characteristics of the river caused by human activities" such as those connected to dams and reservoirs.
Gardner echoed these observations, telling Salon that "I do not believe there are any immediate threats to human health (due to long-term changes in river color)." Although nature already has put dissolved organic matter, sediment and algae in most rivers, "too much or too little (depending on the river) can be harmful for ecosystem and human health." He pointed to the Ohio River in 2015 as an example of a situation in which algal blooms "can decrease oxygen concentrations which fish need to breathe and can also produce toxins." By contrast, "many pollutants (such as mercury) move attached to suspended sediment, but generally in the large rivers included in our study, there is too little suspended sediment which is problematic for coastal wetlands which need river sediment to keep pace with sea level rise."
He emphasized that "we only measured color, which is the combined effect that sediment, algae, and dissolved organic matter has on how water appears to the human eye. Therefore, we cannot exactly say what is changing river color and if it is problematic for human (or ecosystem health)."
Salon also reached out to climate scientists who said that the new study's findings can, in certain cases, be linked to man-made global warming.
"There are a number of human impacts that are leading to these," Dr. Michael E. Mann, a distinguished professor of atmospheric science at Penn State University, told Salon by email. "Runoff pollution in the form of agricultural fertilization can lead to algal blooms (typically cyanobacteria) that turn the water yellow, green or red. But climate change is also a factor. Warmer waters lead to anoxic conditions that favor algal blooms. So in short, the changes that are reported here are a consequence of multiple environmental insults by human activities, including climate change."
Dr. Kevin Trenberth, a Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research at the University Corporation for Atmospheric Research, made a similar point to Salon, writing that "no doubt the dominant source of the changes relates to more people in more places doing more things" like changing how they use land, building dams and changes that interfere with nutrient flows into rivers. He said that "climate change effects are no doubt also in play but are likely somewhat lesser in amount," citing as examples changes in rainfall patterns, the rapidity at which snow melts and the increase in wildfires.