Thanks for the advice so far!

The watershed is coming in from two directions as two small creeks join together just before the spot where we're going to build the dam, so the pond is going to be L-shaped. The causeway is supposed to act like a silt barrier, so the smaller upper pond (about 1ac.) will likely be more of a wetland. Maybe we will look into building a second causeway to filter incoming water from the other direction. One good thing is that almost all of the drainage area is heavily forested and pretty sandy/rocky, so hopefully there won't be too much mud coming in.

In terms of the watershed, our engineer buddy seems to think we'll be ok with a large overflow pipe and a good-sized spillway, but I'm glad to know your concerns about the watershed and adverse effects on fish population. I'm going to bring that up with him and see what we can do about diverting some of that water.

I'm mostly concerned that the dang thing won't hold water because of the shallow bedrock!

Thanks again for the advice - this forum is fantastic.