HENDERSON, La.—Cars wind slowly around Our Lady of Mercy church, into the little city park with its metal-top party pavilion. In happier times there are bands and dances, birthdays and reunions, beneath the metal.

No party today. People are in line for bottled water.

On the Butte la Rose Highway, colorful camps with whimsical names like “Dad’s Pad When Mom’s Mad” line the canal. Imagine a scout jamboree extended for several miles, with laughter and good food, campfires and camaraderie. That’s the usual scene.

No gumbos or cookouts today. Entire families, the...

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