Living near water is a wonderful thing—except when there’s a flood. So people build levees. A levee is a natural or artificial wall that blocks water from going where we don’t want it to go. Levees may be used to increase available land for habitation or divert a body of water so the fertile soil of a river or seabed may be used for agriculture. They prevent rivers from flooding cities in a storm surge. But if a levee breaks, the consequences can be disastrous.

Levees are usually made of earth. The natural movement of a body of water pushes sediment to the side, creating a natural levee. The banks of a river are often slightly elevated from the riverbed. The banks form levees made of sediment, silt, and other materials pushed aside by the flowing water. Levees are usually parallel to the way the river flows, so levees can help direct the flow of the river.

Levees can also be artificially created or reinforced. Artificial levees are usually built by piling soil, sand, or rocks on a cleared, level surface. In places where the flow of a river is strong, levees may also be made of blocks of wood, plastic, or metal. Where the area beside a river or other body of water is in particular danger, levees may even be reinforced by concrete.

People have been building and reinforcing levees since the beginning of civilization. As early as 2500 B.C.E., the Indus Valley Civilization, with urban centers in what is today Mohenjo Daro and Harappa, Pakistan, used levees to protect land near the Indus River. Farmers were able to grow crops like cotton and rice.

In addition to creating living space and cropland, levees can also provide a measure of protection from invaders. Levees can make a river like a moat, preventing people from easily invading territory on the other side. Destroying levees can also stop invading forces. In 1938, Chinese leaders intentionally broke levees on the Yellow River to prevent the Japanese military from advancing. More than 500,000 people, Japanese and Chinese, died in the resulting flood.

Artificial levees need to be protected. They have to stand up to erosion, or wearing away, by the nearby water. Sometimes, trees and plants like Bermuda grass (Cynodon dactylon) are planted near levees to anchor the soil. Engineers need to maintain levees with structural work to reinforce the boundaries.

In emergencies, temporary levees can be made of sandbags. These soak up the water and usually prevent excess water from seeping past the sand.

Artificial levees prevent flooding. But they also create a new problem: levees squeeze the flow of the river. All the river’s power is flowing through a smaller space. Water levels are higher and water flows faster. This puts more pressure on levees downstream and makes the water more difficult to control. If levees break, it also makes containing the flood more difficult.

Since the 18th century, levees have protected Louisiana and other nearby states from flooding by the Mississippi River. When Hurricane Katrina struck the U.S. city of New Orleans, Louisiana, in 2005, the levees could not withstand the storm surge. The levees broke, and water flooded 80 percent of the city.

Levees on the Sea

Although most levees exist to control rivers, they can also exist on the coast. The country of the Netherlands has an elaborate system of dikes, levees, and dams to hold back the North Sea. Land for farms, industry, and residential use has been created from land that was once the ocean floor.

The Bay of Fundy, which borders the provinces of New Brunswick and Nova Scotia, Canada, has one of the highest tidal ranges in the world. The tidal range reaches more than 17 meters (55 feet) in some places. To make the most use of land that would otherwise be underwater during high tide, Canada has constructed levees along parts of the Bay of Fundy.