Our friends at Wikipedia describe it this way:
Carter Lake is a shallow oxbow lake in Nebraska and Iowa, located next to Omaha and Carter Lake in the United States. Soon after its formation the lake was called the East Omaha Lake, and then Lake Nakoma.
Carter Lake is a former channel of the Missouri River and was formed in the summer of 1877. The city of Carter Lake, Iowa takes its name from the lake. The lake was formed from the Saratoga Bend in the Missouri River.
The Saratoga Bend was the impetus for the creation of the town of Saratoga, Nebraska Territory, a short mile from the river. However, the Bend was cut off from the river after a flood in 1877. A beach resort with a large boathouse and two-story pavilion, a Rod and Gun Club, and a YMCA camp had all settled on the shores of Lake Nakoma by 1906.