A levee is a man-made structure, usually an earthen embankment, designed and constructed according to sound engineering practices to contain, control, or divert water flow to provide protection from temporary flooding.
There are 28.2 miles of levees along the Columbia River, Sandy River, and the Lower Columbia Slough system. These levees are made of sand placed on top of Columbia River alluvium (sand, silt, and other materials deposited offshore from the river). They are engineered to allow a small amount of water to seep through the levees into a drainage system that pumps the water away. This allows for flood control while also reducing the pressure on the levee to prevent failure.