This episode takes place in Northwestern Iowa near Sioux City.
Numerous times, they visit a lake, and according to the captions (Which tell where and when the scene takes place) it is Lake Okobogee, Iowa.
This is not the correct spelling. The correct spelling is actually Japanese, Okoboji. This can be verified by looking near the NW corner of Iowa on a map (Dickinson County). Also being a native of Iowa for the past 17 years, this is pretty much a known fact in this area.