A fish kill has grown larger since first being spotted on Saturday in northwest Iowa. The Department of Natural Resources reports heavy rain caused a manure discharge from the Summit Dairy in O’Brien County to eventually reach Mill Creek.

DNR senior environmental specialist Tom Roos says the fish kill now stretches about 12 miles, from near Paulina to the Cherokee County line. It remains unclear how many fish the manure spill has killed. “We’re still in the counting process, but it’s likely going to be in the thousands,” Roos said. The ammonia level at the 12-mile mark is lower than upstream, according to Roos.

Summit Dairy had dug a trench to redirect rain from a construction site, which contributed to the fish kill. “We had a pretty significant rain event, or several, last week and it pushed some open lot runoff and silage leachate into that trench, discharged to a bean field, and ran down into the creek,” Roos said.

It’s unclear what penalties Summit Dairy might face. “That will be determined after we get our fish numbers and laboratory results back,” Roos said. After the DNR traced the incident to Summit Dairy, workers responded by redirecting the runoff to a basin and added a berm to prevent further discharges to the stream. The dead and dying fish include suckers, minnows, darters, shiners, catfish, small largemouth bass, and madtoms.