McCook Lake is a natural oxbow lake found in Union County, South Dakota, United States, about one mile north of North Sioux City. It was formed from a "cutoff" of the Missouri River. The lake is named for General John Cook, who commanded a company of soldiers stationed there in 1862–63 following the Dakota War of 1862. When or why the "Mc" was added is unknown. Most residents are located on the north side of the lake, while land around the south side is used for farming. The Izaac Walton League, an environmental organization active in lake issues, has a clubhouse located on the lake and owns most of the southern shore.