It's levee, not levy. Two very different words.
This is a levee. You may know it as a dike.
Here is a damaged levee.
Here is an aerial photograph of a levee.
Basically, it keeps water in a river. Here is a handy diagram.
We are dependent upon them throughout much of the US to contain rivers like the Mississippi, which are prone to drastic, devastating shifts.
During Hurricane Katrina several levees were damaged, adding to the disaster.