Why is the Mississippi delta sinking?

The Mississippi delta is sinking because the river that built it has been fenced in, so it can no longer drop the mud that keeps the land up. A river delta is a landform, typically triangular, built from sediments a river deposits where it meets slow-moving water such as the sea. It stays above water only while fresh sediment arrives to replace what sinks.

For thousands of years, the Mississippi did that job by wandering. Before extensive leveeing began in the 1930s, it shifted its course roughly every 1,000 to 1,500 years in search of a shorter route to the Gulf, building a delta lobe and then abandoning it. Where the river left, the region lost land through subsidence and erosion. Where it arrived, the land grew, so new land was always being built somewhere.

Then people stopped the music. Most levees went up after the 1927 flood. The river's sediment is the fuel for land growth, but levees block it from reaching most of the delta, and dams and canals in the basin add to the shortage. Without sediment to counteract it, net subsidence in the Mississippi delta is much faster than in other areas of the United States. Researchers suggest that fluid extraction by the oil and gas industry may make it worse.

The stakes are practical. Coastal wetlands and barrier islands are the first line of protection for Louisiana communities from hurricanes. After Hurricanes Katrina and Rita in 2005, about 200 square miles of wetlands became open water. And as sea levels rise, subsiding areas may flood permanently.