Justin,
A dam is nothing more than a strategically placed pile of soil to create a levee, holding back water where it is needed. If your plan is to excavate downward to create a "bowl" that will fill up with the spring water, and then entire perimeter of the bowl-zone is high enough to contain the water, then a dam is not required. This is alot of work, tho. That is why some of the best and most cost-efficient ponds are constructed on a site where the majority of the pond perimeter is naturally high enough (like a valley) and the only areas that need to be corked up to hold the water is where a dam would be placed to tie into the existing valley walls. The digging down to create the bowl is not really required. M Nature provided a bowl with an edge busted off. The dam fills in the busted edge. The soil to create this dam is usually scraped out of the inside of the bowl, close to the area to be corked up.
