For example, a system of dikes hold the North Atlantic Ocean from flooding the Netherlands. Many rivers produce levees naturally during floods when the overflowing river deposits silt and debris along the bank. Gradually this builds up and contains the stream into the channel. Artificial levees are constructed in much the same manner. They may be temporary, as when sandbags are used during flooding, or permanent when the banks are raised to keep the river in its channel during times of increased water flow.
Levees protect the surrounding countryside from floods by holding more water in the channel.
They also aid in navigation by deepening the channel. The Mississippi River has many such levees along its banks but because of its immense size, water is allowed to drain between the levees into low lying land of little value during flooding. Doing this avoids popultaed and farmland as well as avoiding cities and towns. Both levees and dams are used throughout Southern California to control flooding. The San Gabriel and Los Angeles Rivers are good examples where many levees may be seen.