An 8 ft. wide dam is pretty short. I am assuming that you stopped up a natural "draw" of some kind. For a dam to hold, it would have to be good soils on each side. If those assumptions are true, I would get a dozer in and core it; then fill the opening with good sandy loams that contain around 50% clay. It will have to be well packed at each stage. Next, I would make sure that it had a place for excess water to escape, around a side, over natural ground. Make sure the dam isn't the lowest spot. Moisture content of the fill material is also important so it will pack.
Mike Otto has been writing about dams and pond building in PB magazine for over a year. I would study back issues until I had a pretty good understanding of soils and dams.
