This is Idaho’s famous “River Between the Lakes” in Heyburn State Park. It is and isn’t a natural feature. Prior to installation of the Post Falls Dam in 1908, Hidden, Round, Benewah, and Chatcolet were individual lakes, except during high water when they merged with the larger Lake Coeur d’Alene. The dam holds the lake at is high-water level, so the St. Joe River, which winds between the southern lakes, has now all but disappeared, leaving just a thin strip of bank on either side through Heyburn State Park.
For more Idaho state parks history, read Images of America, Idaho State Parks by Rick Just. Now available online and in your local bookstore.