Land in Southern Louisiana was built over thousands of years by the sediment carried by the flow of the Mississippi River.

Sediment that had eroded and flowed through tributaries distributed throughout the entire Mississippi River basin was deposited along its lower reaches and outlets in the Gulf of Mexico, building the Mississippi River Delta.

Banner image from NASA Earth Observatory

As the river built land its path was steadily impeded and the slope of its channel reduced. This resistance to flow caused avulsion and the river periodically switched course, creating whole new deltaic lobes. Thus as the river continued to build land in its new alignment, previous lobes were abandoned and no longer had the supply of sediment to sustain them.

Slowly these abandoned lobes shrank as subsidence, wave and storm action swept previously built land into the sea. In this way the Mississippi River Delta has always been simultaneously growing in some areas and shrinking in others.