I'm not really so sure, but it probably has something to do with size.
I see a water hole is defined as:
1. A geological or natural depression in which water collects, especially a pool where animals come to drink.
2. A social gathering place, such as a bar or saloon, where drinks are served (chuckle).
A dam is defined as:
1. A barrier constructed across a waterway to control the flow or raise the level of water.
(With other words, it is a construction (the wall) that is built across a natural flowing waterway in order to accumulate water or to control the flow of water.)
2. A body of water controlled by such a barrier.
So in the context of our nature reserves or parks, I would venture to say that a dam is a man made construction built with the sole purpose to accumulate water and a waterhole is a natural occurring body of water.
Then of course you also find pans in our parks.
These are small concrete pools with windmills or solar panels & electrical pumps to pump water out from deep within the earth.
Nothing Beats Fresh Fish (110) *