They’d built all this after the terrible flood of 1927, which had inspired the blues song "When the Levee Breaks." It was intended as a safety valve, a bypass to be opened at rare moments of dire need, when heavy rains falling upstream—which was to say anywhere between the Rocky Mountains and the Alleghenies—had swollen the river to the point where it was threatening to overtop or burst through the levees and inundate New Orleans. Several times in the ensuing ninety years it had been used just as planned. But then in 2019 they’d been forced to leave it wide open for four months because of a wet winter. Other years since had not been quite as bad as that, but 2019 had inaugurated a new pattern, which was that the spillway was partially open more often than not.