For years the Klamath River came with a warning label every summer, signs telling people not to touch water that could kill a dog, thanks to blue-green algae blooming in the reservoirs upstream. Two years after those reservoirs drained, the toxin is barely showing up at all. The Angry Optimist, a nature and science newsletter, compiled the latest data from the Karuk and Yurok tribes, Sustainable Northwest, and CDFW. Below the former Iron Gate Dam, where 58 percent of Karuk samples once exceeded the public health limit for microcystin, every sample since removal has come in under the limit, and in 82 percent the toxin could not be detected at all. The reason traces back to what the four reservoirs actually were: roughly 2,200 acres of warm, still, nutrient-rich water, exactly what Microcystis needs. 

Related: 

• The Angry Optimist: Blog: Toxic algal blooms nearly gone from the Klamath River