However, an unintended consequence of the levees was that natural silt deposits from the Mississippi River were unable to replenish the delta, causing the coastal wetlands of Louisiana to wash away and the city of New Orleans to sink even deeper. The Mississippi River delta is subsiding faster than any other place in the nation. While the land is sinking, sea level has been rising. In the past 100 years, land subsidence and sea-level rise have recently added three feet to all storm surges. That extra height puts affected areas under deeper water; it also means flooding from weaker storms and from the outer edges of powerful storms spreads over wider areas. The marshes that ring New Orleans, as well as the land depression of the city itself, originally above sea level, have sunk the quickest.