"A fertilizer spill in Iowa this month wiped out much of the aquatic life across a 60-mile stretch of rivers in two states"

"The latest die-off started, Iowa officials said, when a valve was left open over a weekend on a storage tank at ... an agricultural business" 

(The article is a choose-your-own adventure with an early quote calling it "the big one," and a later paragraph saying, "But the biomass affected is likely pretty low as the kill was mostly minnow and chub species.")