Short answers, pulled from the story.
A levee is an elevated ridge of earth built alongside a river or coastline to prevent flooding of the surrounding land. Artificial levees are broad at the base and taper toward a level top; their surfaces are planted with vegetation such as Bermuda grass to resist erosion, and stone or concrete revetments are installed on the river side where wave action is strongest.
French settlers in Louisiana began building the first Mississippi levees in the 18th century to protect New Orleans, with initial structures about 3 feet high covering roughly 50 miles. The U.S. Army Corps of Engineers extended the system from 1882, and by the mid-1980s the levees reached their present extent of over 3,500 miles, averaging 24 feet in height.
Levee failures are caused by overtopping, surface erosion, structural weakness, or saturation of the earthen body. The most dangerous failure is a breach, where a section breaks open entirely, leaving a gap through which floodwater floods the protected land. The 2005 levee failures in Greater New Orleans during Hurricane Katrina are among the most catastrophic examples on record.
Natural levees form when a flooding river loses velocity as it spreads across the floodplain, causing coarser sediments to drop near the channel edge and finer particles to settle further out. Over many flood cycles these deposits build up into ridges on either side of the channel. Rivers with high suspended sediment loads, including tidal creeks affected by high spring tides, are most likely to develop natural levees.
The Indus Valley civilization built levees from around 2600 BCE in what is now Pakistan and northern India. Ancient Egypt constructed a levee system more than 600 miles long along the left bank of the Nile, stretching from the area of modern Aswan to the Mediterranean. In the early 1400s, Nezahualcoyotl of Texcoco supervised a levee to separate the brackish and fresh waters of Lake Texcoco to protect Mēxihco-Tenōchtitlan.
Artificial levees block the sediment, nutrients, and organic material that floods would otherwise deposit across floodplains and wetlands, reducing riparian biodiversity. By concentrating river flow, levees also intensify flood pulses during extreme storm events. Studies of areas where levees were removed found increases in riparian habitats and biodiversity afterward.