Toxic blooms form for a variety of reasons, including warm weather and stagnant water. While they’re most often associated with lower-elevation reservoirs and places where nutrient runoff from agriculture, fertilizers, urban areas, wastewater treatment plants…or septic systems is high, they don’t exclusively form in areas with direct runoff.

For reasons researchers are still trying to parse, the U.S. Forest Service is discovering more of them in high-mountain lakes, which could be attributed to atmospheric pollution or cycling of nutrients from forest fires or even beetle-kill trees.