Either rent or have someone come in with a self loading pan scraper. What it will take you a year and a lot of fuel in a dozer you could do in a long weekend. I did a job site and dug an acre pond 6' deep, for water retention, stripped 2 acres of top soil and stacked it 1/4 mile away and used sand base of pond as fill for the 1.5 acre building site and parking area in 7 days. BE WARNED, the scraper burns 100 gl diesel a day and is the roughest machine you will ever operate.
I have never built a dam but if you have some nice heavy clay, there is nothing that packs in better than a pan scraper full of dirt.
Buy a transit if you do not have one. You will need it to get your dam level and get the fall right for your spill way or taper it off and make it more like a creek run off
