The Mississippi River at New Orleans has never been so high for so long.

ARABI, Louisiana—The water is breaking gently around the hull of the crew boat Miss Emerson, as if she were puttering across a muddy lake. In New Orleans, the Mississippi River is considered to be at “high water” when it runs through the city at more than 8 feet above sea level. The river might not run at depth through New Orleans at all if it weren’t for the Army Corps of Engineers’ maintenance of the Old River Control Structure, a system of floodgates that is the subject of McPhee’s famous 1987 essay “Atchafalaya.” Old River Control ensures the Mississippi remains deep enough in Baton Rouge and New Orleans to support shipping on the lower river. “It’s the first time it’s ever happened, but I wouldn’t say it’s unusual.” In New Orleans, you can go days without thinking of the river. “One is trapping water in natural floodplains up north, providing space for water to go before it even gets to the river.” The other is changing the course of the lower river below New Orleans, which would both spread sediment onto subsiding coastal marshes and reduce the backup caused by the river’s tortured route to the sea.