DAKOTA DUNES, S.D. — Some skeptical locals offered a warning when developers transformed this mostly barren peninsula at the intersection of two rivers into an exclusive planned community, complete with million-dollar homes and a private golf course designed by Arnold Palmer.

They call it “the Dunes” for a reason, the warning went — the rivers put the sand there, and the rivers could sweep it away. But, much like the developers, the new residents were not worried. A few even paid a premium to be closest to the flowing water of the Missouri and the Big Sioux.

Now, a little more than two decades later, the stately homes on Spyglass Circle and Pebble Beach Drive have been evacuated and the 18th hole is under six feet of water, as miles of newly built levees strain to keep this community from surrendering to a historic flood.

Many residents here at the southeastern tip of the state, where it borders Nebraska and Iowa, say they never imagined this chain of events. Scott Mackie, like most of his neighbors, did not take out flood insurance on his newly built house — not because he could not afford it, he said, but because he believed the Missouri had been tamed by a system of dams and reservoirs.