A few miles short of New Orleans, he came to the Bonnet Carré Spillway, which was not much to look at but was probably at the top of the list of must-see attractions for the climate change tourist. Here all the roads and railways had been funneled into the five-mile-thick isthmus separating the Mississippi’s left bank from Lake Pontchartrain. Running athwart all that was a mile-wide sluice connecting the two. Or rather it was capable of doing so when it was opened. A hundred years ago they’d constructed a spillway barring its Mississippi end. This was a palisade consisting of seven thousand massive vertical timbers arranged vertically in a frame of concrete and steel, deeply anchored in the river’s bank. To one side, these were scoured by the flow of the Mississippi. To the other side was what passed for dry land: a wetland stretching from there to the lake, striped with drainage ditches and speckled with puddles and ponds, passing beneath elevated highways and railroads.