Rivers across the U.S. are changing colors. A new study reveals why.

Striking new images reveal that a third of U.S. rivers have significantly changed color from blue to yellow and green, over the last 36 years, The Weather Channel reports.

With help from NASA and the U.S. Geological Survey’s Landsat program, a research team led by John Gardner — a postdoctoral researcher in the global hydrology lab at University of North Carolina — analyzed 235,000 satellite images taken over a 34-year period to compare hue changes in hundreds of rivers across the U.S.

Their findings revealed that more than 50% of the rivers had a dominant hue of yellow, more than a third of the rivers were green and just 8% were blue, according to Live Science.

A river’s color depends on how much algae and/or sediment it contains, according to the site. As a general rule, river water turns green as it carries more algae blooms and water tends to turn yellow when it’s carrying more sediment.

“Sediment and algae are both important, but too much or too little of either can be disruptive,” Gardner told Live Science.

According to the team’s recent study, published in Geophysical Research Letters, only 12% of the rivers surveyed maintained a constant color over the 34 year period, but that shouldn’t necessarily raise any concerns, according to The Weather Channel.

“Big trends to yellow or green can be worrying,” Gardner said, but added that “it depends on the individual river.”

Much like leaves in autumn, rivers can change colors depending on the season, The Weather Channel reports. Rivers in the western U.S. tend to adopt a more yellow hue in the summer as snowmelt carries new sediment into waterways.

The satellite images did reveal clear color changes that came as a result of human influences, however, like dams, reservoirs, agriculture and urban development. According to Gardner, these changes aren’t necessarily permanent.

“You could totally see these trends going back in the other direction,” he said, “especially if the change is occurring due to local mismanagement that is easily fixed.”

Gardner admits that, while color changes provide an easy, visible way to monitor shifts in a river’s ecosystem, color change isn’t necessarily an accurate barometer of a river’s health.

“Because large rivers integrate millions of kilometers of land area, understanding rivers and their impairments is inherently macroscale: both distant and local impacts generate the patterns we observe,” Gardner’s research team wrote, according to The Weather Channel. “There is a profound need for integrative water quality measurements.”

Researchers are now using the color changes as a starting point to conduct more precise surveys regarding the health of specific river ecosystems across the country, Live Science reports.

You can learn more about how your local rivers have changed over time by exploring this interactive map, created by Gardner and his team, per Live Science.