The earliest-settled parts of New Orleans and surrounding communities are above sea level. However, flooding was long a threat, from the periodic high waters of the Mississippi and from more occasional severe tropical storms which pushed the waters of Lake Pontchartrain into settled areas. Construction of the levees along the River began soon after the city was founded, and more extensive river levees were built as the city grew. These earthen barriers were erected to prevent damage caused by seasonal Mississippi River flooding. The Lake Pontchartrain shore was mostly undeveloped swamp, and only small levees were built there in the 19th century.