Biologists from the Iowa Department of Natural Resources (DNR) are investigating a large fish kill in O’Brien County that was discovered on Tuesday. According to the DNR, the incident was caused by a manure spill from Summit Dairy near Mill Creek in the upper Mississippi River. Officials confirmed that the fish kill has spread 12 miles to the edge of Cherokee County and is reportedly one of the largest Iowa fish kills in recent memory.

“The runoff caused an elevated level of ammonia and lowered dissolved oxygen in the creek,” said DNR environmental specialist Tom Roos in a press release. “At the 12-mile mark the ammonia level is beginning to drop, but we’re still seeing dead and dying fish downstream.

Officials believe that the spill was caused by heavy rains that overwhelmed the Dairy Summit facility, causing runoff to flow into a nearby soybean field and then downstream. The facility has since set a berm to prevent further overflow and redirected some of the runoff to a basin. The manure in the river, however, is still killing fish. Fisheries personnel have reported seeing dead minnows, catfish, largemouth bass, shiners, and other species. The DNR has not yet released an estimate of how many fish were killed as a result of the spill.

“We’ve been on a number of fish kills during my tenure with the department, this is probably one of the larger, lengthier fish kill investigations that I’ve been involved with,” fishery biologist Mike Hawkins told ABC9 News.

Hawkins says that he and other biologists will get to work taking sample counts of dead fish from each species, which will eventually give the DNR an idea of how many fish have died. It is dirty work, and the combination of summer heat, dead fish, and manure can spell unsavory—even harmful—conditions. Officials are currently warning residents to keep livestock and pets away from the affected waters due to health concerns.

Image courtesy Spencer Neuharth / US Fish and Wildlife Service

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