Losing Louisiana
Sea-level rise, subsiding lands in the delta will claim 10 percent of state by 2100
By Sid Perkins
Residents of Louisiana, take note: If engineers don’t divert sediment-rich waters from the Mississippi River to help replenish a sinking river delta, about 10 percent of your state will slip beneath the waves by the end of this century. However, even if the engineers do try to abate the subsidence, the Mississippi doesn’t carry enough sediment to offset more than a small fraction of that loss, a new analysis suggests.
Over the past few centuries, about a quarter of the wetlands in the Mississippi River delta have been lost to the ocean, says Harry Roberts, a marine geologist at Louisiana State University in Baton Rouge. Several factors have contributed to that loss, he notes, including sea-level rise and the settling of land as ancient sediments gradually become compacted under their own weight. Now, Roberts and colleague Michael Blum — now at the ExxonMobil Upstream Research Company in Houston — use computer models to estimate the effect that these processes will have on the Mississippi delta in the next few decades. The news, reported online June 28 in Nature Geoscience, isn’t good.
Tidal gauges at Grand Isle, La. — near the tip of the Mississippi delta, where river-dumped sediments lie about 60 meters thick — indicate that land there is sinking as much as 8 millimeters each year. At Baton Rouge, about 250 kilometers upstream, sediments are thinner and the land subsides up to 3 millimeters per year. In Roberts and Blum’s new analysis, regions between those points sink at intermediate rates.