A pond is a small, still, land-based body of water formed by pooling inside a depression, either natural or artificial, that is smaller than a lake. Defining them to be less than 5 hectares (12 acres) in area, less than 5 meters (16 ft) deep, and with less than 30% emergent vegetation helps in distinguishing their ecology from that of lakes and wetlands.: 460  Ponds can be created by a wide variety of natural processes (e.g. on floodplains as cutoff river channels, by glacial processes, by peatland formation, in coastal dune systems, by beavers), or they can simply be isolated depressions (such as a kettle hole, vernal pool, prairie pothole, or simply natural undulations in undrained land) filled by runoff, groundwater, or precipitation, or all three of these. They can be further divided into four zones: vegetation zone, open water, bottom mud and surface film.: 160–163  The size and depth of ponds often varies greatly with the time...