Levees are natural ridges of sand and silt along the banks of a river. Rivers carry lots of sediment and can carry even more during fast-flowing floods. When a river overflows its banks during a flood, the water slows down and drops heavy sediment particles along the edges of the channel.
Smaller, lighter particles are carried farther out into the floodplain.
Each flood leaves a layer of sediment along the channel edges, gradually
forming the ridged levee. Levees are usually the highest land in the river's
floodplain. They are home to plants such as black willow and American sycamore
that can tolerate seasonal flooding.