A study that analyzed more than 230,000 NASA satellite images over a period of 35 years has revealed major changes in the color of rivers throughout the United States. And, concludes the study published in the journal Geographical Research Letters, it is a potential “warning of environmental change.”

The study says tens of thousands of mile-long river segments, about one-third of flowing rivers in the U.S., have changed color since 1984. An astounding 11,629 miles of river have become greener, indicating an increased presence of algae.

The chief cause of the change, says the study, is farm fertilizer run-off that is known to ignite algae blooms that cause oxygen loss and, as is the case with blue-green algae, produce dangerous toxins.

Martin Doyle, head of water programs at Duke University, says the study “shows how most every aspect of our planet is being affected by humans, now including the basic color of our water. That’s pretty profound if you think about it.”

Most of the shirt to greener rivers has occurred in the North and West U.S. More yellowing of rivers, primarily due to soil erosion, is most prevalent in the Eastern U.S. and along the Mississippi River.