The Minnesota Pollution Control Agency is investigating a fish kill in Renville County.
Authorities were called Wednesday night about dead fish in West Fork Beaver Creek south of Danube.
Observers found dead northern pike, smallmouth bass, suckers and minnows in the creek.
The MPCA says the level of dissolved oxygen in the creek had dropped well below that needed to sustain aquatic life. Investigators don't believe there were toxic chemicals in the creek.
Friday, Southern Minnesota Beet Sugar Cooperative near Renville reported that a valve in its wastewater treatment system had been left open Wednesday night. That resulted in untreated water being discharged into a ditch that goes into the creek.
The co-op is working to install treatment measures, such as placing pumps to aerate water in the stream.