American rivers are changing colors, according to a study published in Geographical Research Letters earlier this month. Researchers aren’t quite sure why the changes are taking place. It’s possibly a sign of the end times, but more likely due to human activities.
To reach their head-scratching finding, scientists compiled satellite images, taken by NASA and the U.S. Geological Survey, from 1984 to 2018 of tens of thousands of river stretches across the country each measuring over 1.25 mile (2 kilometers) in length. Together, the images covered 67,000 miles (10,800 kilometers) of water. They found that over that time, about a third of that combined distance has noticeably changed in hue. We’re not just talking shades of cerulean and teal, either. In fact, the authors found that only about 5% of the 11,629 miles (18,715 kilometers) of river they measured were blue in 2018. A full 56% appeared yellow, though, and 38% had greenish hues. If this is hard to visualize, check out this interactive map the authors made to illustrate their findings.
Rivers do tend to change color temporarily due to seasonal shifts their chemistry. But the shifts the researchers documented were long-term and “significant,” indicating the water bodies in question have undergone a major change.
The authors don’t know exactly what caused each change in color they observed, but they believe many were the result of human action. For instance, the study found that many rivers in the northern and western U.S. turned, which likely indicates that they became full of algae. Said algae is likely tied to runoff from agricultural fertilizer used in those regions, which can cause harmful algae blooms. By contrast, rivers in the eastern half of the U.S. as well as the tributaries of the Mississippi River most often turned yellow, which indicates the presence of a lot of sediment and is probably sign of soil erosion piling up into the water.
There are other factors, including nearby urban pollution or dams, that can change rivers’ colors. Changes in the climate can produce color shifts, too. Increases in sediment, for instance, can also be caused by an uptick in rainfall, and warmer water is more susceptible to algae growth, and both are well-documented symptoms of the climate crisis.
The authors stressed that a change in a river’s color doesn’t necessarily indicate a change in ecosystem health. But the widespread shifts are still quite troubling. After all, more algae in rivers can be harmful to fish and other marine life, and previous research shows that these blooms have become far more common in recent decades. Soil erosion into waterways can also create unsafe conditions for some underwater creatures, and poses a problem on land, too, since degraded land is more vulnerable to flooding and less suitable for growing crops. The study is yet another reminder that human actions—particularly actions by powerful corporations—have changed our landscapes in bizarre and potentially dangerous ways.