A hole dug out of flat ground would not necessarily have a dam. Many ponds that are dug on flat ground would be ground water ponds and fill up because of the high water table, and these ponds are often bermmed all the way around with he soil that is dug out of the hole just to have a convinient place to put it. This would resemble a dam, but is really a berm. These particular ponds are also ususlly lined with a liner and filled by a pump and well or nearby water source.

A dam by definition is meant to stop the flow of water and create a pool...that can not be done on perfectly flat ground without it becoming a full perimeter berm and then it would only collect rain water.

Fish on!,
Noel