National Public Radio reports the Des Moines River reached a temperature of 97 degrees Fahrenheit during the height of the heat wave that gripped the U.S. Midwest this summer.
Aaron Woldt, a fisheries official with the U.S. Fish and Wildlife Service, told NPR that fish die-offs are a natural occurrence but extreme heat reported in the United States this year increased the level of reports.
"If the water heats up, it holds less oxygen and ... fish will start to die from lack of oxygen," he said.
Scientists working for the National Aeronautics and Space Administration found that extreme weather events like this year's heat wave can't be explained by natural phenomena alone, says James Hansen, author of a report on climate.
"For the extreme hot weather of the recent past, there is virtually no explanation other than climate change," he wrote in a recent column in The Washington Post.
The report, to be published in the Proceedings of the National Academy of Sciences, said that extreme weather events may become commonplace within the next 50 years if carbon emissions continue to increase.