Much like the sky, rivers are rarely painted one color. Across the world, they appear in shades of yellow, green, blue, and brown. Subtle changes in the environment can alter the color of rivers, though, shifting them away from their typical hues. New research shows the dominant color has changed in about one-third of large rivers in the continental United States over the past 35 years.

“Changes in river color serve as a first pass that tell us something is going on nearby,” said John Gardner, the study’s lead author and a hydrologist at the University of Pittsburgh. “There are a lot of details to parse out on what is causing those changes, though.”

The figure above shows data from the first map of river color for the contiguous United States. The rivers are colored as they would approximately appear to our eye. Gardner and colleagues built the map from 234,727 images collected by Landsat satellites between 1984 and 2018. The dataset includes 67,000 miles (100,000 kilometers) of waterways of at least 200 feet (60 meters) wide. Around 56 percent of rivers were dominantly yellow over the course of the study and 38 percent were dominantly green. The team has released an interactive map where the public can further investigate color trends in individual rivers.