One-third of American rivers show increasing pollution problems since 1984, according to a systematic new study of satellite imagery displaying their changing color over time.

The red flag in this case is increasing green color in over 11,000 miles of river segments, a sign of growing algae in the waters. As North Carolina water pollution fighters know, this usually means the water is oversaturated with contaminated runoff or other wastes that promote conditions which drain oxygen from the water, killing fish.

Lead study author John Gardner, a geology and environmental sciences professor at the University of Pittsburgh, said “If things are becoming more green, that’s a problem.” Co-author Tamlin Pavelsky, a global hydrology professor at the University of North Carolina, confirmed these changes are related to human activity.

The study, co-authored by six scientists from various institutions and published in this month’s scholarly journal Geophysical Research Letters, was based on a review of satellite imagery taken from 1984-2018 of over 100,000 kilometers of river segments more than 60 meters wide in the contiguous United States. In the plain language summary, as the study points out, “Rivers can appear different colors such as blues, greens, browns, and yellows. Water color is linked to water quality and can be related to the amount of sediment, algae, and dissolved organic carbon in water. Humans can therefore discern waters’ suitability for use with our eyes.”