Subsidence and Erosion in the Lake Pontchartrain Basin Mississippi River sediment has formed most of Louisiana's coastal wetlands over the past 5000 years or more. This happened when river water regularly deposited sediments over land areas during the river's annual floods, accumulating thick layers of sand, mud, and silt. Over time, a series of overlapping deltas formed, eventually building a wide deltaic plain that forms Louisiana's coast. Lake Pontchartrain was formed when Mississippi River deltaic deposits partially closed it off from the Gulf of Mexico. All the land in the Pontchartrain Basin south of Interstate 12 consists of Mississippi River sediments deposited over time.
Fresh sediment is needed to nourish the wetlands vegetation and to insure the buildup of land.
This was never a problem, because periodic flooding of the Mississippi River spread millions of tons of sediment across coastal marshes every year. This natural process of sedimentation might have continued indefinitely if it were not for human intervention. The annual flooding caused problems for people who had settled along the river and its distributaries. This resulted in a system of flood control by building levees to help keep the Mississippi River, as well as rivers and bayous throughout the Lake Pontchartrain Basin, confined to narrow channels. While the levee system prevents damage to people and property, including loss of lives, it also prevents river water from flowing through the wetlands.