By Sid Perkins
About 20 kilometers southwest of New Orleans, one of the U.S. Geological Survey’s benchmarks sits atop a concrete column that pokes above the waves about 5 meters from the shore of Couba Island. The small brass disk, one of thousands that the agency has installed throughout the country, serves as a reference point for surveyors and mapmakers. Why did agency personnel place what should be a readily accessible guidepost in the thigh-deep waters of a Louisiana bayou? The answer’s easy: They didn’t. When the disk was installed in 1932, it sat high and dry in someone’s backyard.
Several factors have contributed to making the benchmark’s locale wet. In recent years, the sea level has risen slowly but surely. Just as gradually, the land that the benchmark sits upon has subsided—in part because of the extraction of oil and natural gas from strata beneath the bayou. But another important cause of the ground sinking is the waning of sediment deposition by the Mississippi River. Between the land slowly sinking and the water rising, Louisiana each hour is losing ground equal to two football fields.