The state in which the story is set is never revealed. We know that it is somewhere in the Midwest. Because of the mention of Cedar County, the Masterplots summary speculates that the setting is Wisconsin, but there is a Cedar County in Iowa, Nebraska, and Missouri as well. All we can say for certain is that the story takes place on the prairies of the Midwest.
Even though, the story never reveals the state, it talks about a city, Clear Lake. We know that this story takes place in the Midwest, and when researched, Clear Lake is in Iwoa.