A levee is a wall or embankment built along a river or coastline to prevent flooding. When rivers overflow their banks during heavy rains or snowmelt, levees hold back the rising water and protect nearby towns, farms, and homes.

Levees are typically made of earth, concrete, or a combination of materials, and they can stretch for miles along a waterway. The Mississippi River has an extensive system of levees protecting cities like New Orleans. Ancient civilizations built levees thousands of years ago: the Mesopotamians constructed them along the Tigris and Euphrates rivers, and Chinese engineers built levees along the Yellow River.

Building and maintaining levees requires careful engineering. If a levee breaks or is overtopped (when water rises higher than the levee itself), the results can be catastrophic. Water rushes through the breach with tremendous force, flooding everything in its path. Engineers must constantly inspect and reinforce levees, especially as rivers change course over time or as development increases behind a levee.