A lightning storm knocked out power to a pumping station at West Okoboji, causing sewage to back up, officials said.
The storm on Sunday also knocked out a fuse that prevented an automatic telephone dialer from alerting officials from the Iowa Great lakes Sanitary District.
Sewage ran onto the sand at Manhattan Beach and affected numerous rental units.
The amount of sewage that spilled had not been determined, the Department of Natural Resources said. The spill affected only the immediate area around the beach and shouldn't have a serious impact on the lake, the DNR said.