If you want to use sacks, consider the 40 pound ones. The edge of the sack is what you really want to protect the dam, so there's no advantage to wider sacks. You can get more edge area for your money with the 40 pound sacks versus the 80 pound ones.
The real work will be digging a base to stack them on. Once you have that, then just place them where you want them and pound some rebar through the sacks. The rebar goes through the sacks real easy, but when they set up, the rebar will lock them all together. You probably want at least one row below waterlline, then two above.
Another option might be blocks. Landscape retaining wall blocks might be cheaper and allot nicer looking.
But before I did either of those, I'd price rock. Some river rock, or something that's available locally would probably be cheaper, easier and just as effective.
Eddie
