In Louisiana five thousand square kilometres of land have been lost since 1932
The US geological survey has measured 5,197 square kilometres of land gone from the coastal parishes between 1932 and 2016. The state was built with the mud the river brought down, and it stopped receiving it the day the river was channelled.
Almost every territory we visit looks permanent on a human scale. Not this one. Here there are eighty-year-old maps that no longer work for navigation, and towns that used to appear on dry land and are now surrounded by water.
And what is editorially interesting is that the main cause is not the sea rising, but an engineering decision taken upstream a century ago.
01 Five thousand one hundred and ninety-seven
The figure comes from work by the US geological survey comparing the land area of the coastal parishes between 1932 and 2016. The result is 5,197 square kilometres lost, plus or minus 443.
The rate has not been constant. Measured rates run from 83.5 square kilometres a year at the worst moments, around the mid-1970s, to some 28 a year in the latest stretches of the series. The loss continues, but more slowly.
The comparison used locally is an American football field: depending on the period, one has been lost every thirty-four minutes at the worst point, and one every hundred minutes at the most recent rate. It helps convey the scale, though the figure that matters is the first one.
02 A state made of borrowed mud
To understand the loss you have to understand what this land is made of. The southern half of the state is not rock: it is sediment the river brought from half a continent and dropped on reaching the gulf, layer upon layer, over thousands of years.
A delta works like a conveyor belt that keeps changing position. The river floods each spring, lays silt on the side plains, eventually chokes its own channel and then breaks through somewhere else, starting to build a new lobe. The old one is left unfed and sinks slowly.
That cycle of building and abandoning is what raised the entire coast. The state covers 135,382 square kilometres, of which 23,102 are water: fifteen per cent. That is not a quirky geography fact, it is the definition of the place.
03 The day the river stopped flooding
Here is the turn. Over the 20th century the river was channelled between continuous levees to protect cities and farmland from floods, and to keep a reliable navigation channel open. It worked: floods stopped sweeping the plains.
But those floods were precisely the replenishment mechanism. If the river does not overflow, the silt is not spread across the marsh: it travels locked between the levees to the mouth and is lost in deep gulf water, where it builds nothing.
The result is a delta that keeps losing material from below and from the sides but no longer receives replacement from above. Add to that the thousands of kilometres of straight canals cut through the marsh for navigation and industry, which let salt water inland and kill the vegetation holding the soil together.
04 And the ground is sinking too
The second half of the problem is that a delta sinks on its own. Freshly deposited sediment is loose and full of water; over time it compacts under its own weight and loses height. In a healthy delta, that drop is offset by new material every year.
When the supply is cut, compaction carries on alone. The ground drops a few millimetres a year, and on a plain that sits a few centimetres above sea level, a few millimetres are the difference between marsh and lagoon.
That is why the loss does not look like a retreating cliff but like a marsh fraying. In aerial photographs you recognise it at once: the grass breaks into shreds, ponds appear inside it, the ponds join up and what was a continuous meadow ends up a bay.
05 The one place that is growing
There is one exception, and it is the one that proves the argument. In the south-centre of the state, the Atchafalaya basin is the only area with a growing delta system and wetlands that are almost stable, while the rest of the coast comes apart.
The reason is that sediment does arrive there. That basin receives a diversion of the main flow and works the way the whole delta used to: the water spreads across a wide surface, drops its silt and builds new land at the mouth.
It is also the country's largest swamp wetland, with around seventy per cent forest and thirty per cent marsh and open water, and it holds the largest continuous block of flooded forest left in the lower valley. It is, literally, the witness of what used to be.
06 How to travel it
The first decision is accepting that none of this is visible from the road. The coast has no viewpoints and no cliffs: it is a plain of grass at water level, and to understand it you have to get into a boat or a light aircraft and look from above.
The second is choosing the two ends of the story. On one side, the marsh being lost, to the south and east; on the other, the inland basin that keeps growing, with its flooded forests. Seeing both on the same trip is what turns a swamp outing into a geography lesson.
And the third is a way of looking. When you cross a canal as straight as a ruler in the middle of the marsh, look at its edges: they are almost always more broken up than the rest. That canal is one of the reasons the count of 5,197 square kilometres keeps rising.
