When $1 billion isn’t enough. Why the Sioux won’t put a price on land.

It’s been decades since Madonna Thunder Hawk last saw the valley she grew up in on the Cheyenne River Reservation. It lies buried under the Missouri River. The United States government sent the river rushing over the reservation’s largest town in 1960 as part of a series of post-war federal flood control projects.

In the history of Native American land dispossession in North America, the creation of the Oahe Dam is little more than a footnote. But for Ms. Thunder Hawk, it is her footnote. She couldn’t bear to...