Wetland
A constructed wetland is man-made to make a piece of land hold more water than it otherwise would naturally. This allows the land to stay wet enough to grow wetland plants and oftentimes hold water as well. Uses include absorbing flash floods (by holding the extra water from the flood), cleaning sewage (which is done by the plants as the dirty water passes over their roots), enhancing wildlife habitat (for example creating new homes for rare birds or other animals) or for some other human reason.