You might not be able to tell with a casual look, but rivers across the U.S. have been changing color. And that’s not necessarily a good thing.
A recent study shows that of the tens of thousands of mile-long river segments across the country, approximately a third have noticeably shifted color in satellite images during the last 35 years. More than 11,000 miles became greener or moved toward the violet end of the spectrum, and people are the cause of many of the shifts.
Green rivers, which now account for 28 percent of river segments, are an indication of substantial algae issues. Only about 5 percent of U.S. rivers are classified as blue, which is often equated with pristine waters by the public. About two-thirds of American rivers are yellow, which indicates they have lots of soil in them.
Researchers say the growing rate of green coloration in rivers is a problem because it tends to signal harmful algal blooms (HABs) that can produce toxins. These toxins — typically caused by farm fertilizer runoff, dams, efforts to fight soil erosion, and climate change — increasingly pose a threat to municipal water supplies.
The study looked at more than 230,000 NASA satellite images over three-and-a-half decades, focusing on rivers and reservoirs. The study found much of the shift to greener rivers happened in the North and West, while the yellowing occurred more in the East and around the Mississippi River. It also found some rivers change colors naturally with the seasons.
Multiple organizations across the globe are working to combat the HAB problem.
A seven-year analysis of almost 10,000 HAB events worldwide over three decades was recently set to be published by the HAB Programme of UNESCO’s Intergovernmental Oceanographic Commission.
More than 100 scientists in 112 countries contributed to the synthesis and analysis of HAB data gathered from 1985 to 2018 — a first-ever big data approach to detecting changes in the costly phenomenon’s global distribution, frequency, and intensity.
In the U.S. specifically, groups such as American Rivers combines national advocacy with field work in key river basins. Among its goals, American Rivers is working to protect 5,000 new miles of wild and scenic rivers and one million acres of riverside lands; restore damaged rivers to improve flows, reconnect fish, and wildlife habitat and improve public safety; remove 400 hazardous and outdated dams; restore 10,000 miles of rivers and 1,000 acres of floodplains; and reduce pollution in 100,000 miles of rivers.