For millions of years, the Mississippi River flowed unchecked, carrying roughly 400 million metric tons of sediment down to Louisiana, where it spilled into the Gulf of Mexico to create new land. But in the early 20th century, a series of dams and river-training structures were built to prevent flooding — leaving the river tamed and unable to produce new terrain at anywhere near its previous pace. Oil and gas development, which ripped broad canals through vulnerable marshland, made matters worse.