Thanks to all who provided advice and suggestions! The project has been successfully completed and results can be seen here:
creek dam 10 bags were needed. Bob was right - after the first attempt the pond only filled about halfway. Some leaked through or under and much went around one side. One bank is near a tree and too much water was flowing though its root network. I dug this out some, trimmed some roots, and installed the plastic liner along this side and in front of the dam. I also dug a little deeper in the center and used this fill to cover and weight the liner. The next day, water was flowing over the top at the intended spillway and the pond was full. It pooled up about 25 ft. back from the dam, which was larger than I had hoped. Quite a nice little pond - thanks again!
(I'm curious to see what happens after we get a good toad-choker.)
On another note, I checked with ADEM about permitting and they said a project of that size wasn't a concern to them, but that I could write the ACE district and ask for a letter stating that such a lansdcaping project would not need a specific permit.
