Matt,
From the topo map, I'd put the spillway on the other side. It looks more shallow to me, and the slower the water moves, the less erosion that it causes.
As for building the dam across the creek, the issue is obviously the water in the creek and what you have to work with. Digging the key way would be best with an excavator, but a backhoe will get it done too. Can you cross the creek with equipment? Can you pump or syphen the water over the dam?
I hate the thought of a pipe through the dam, but if the water is flowing pretty good, that might be your only option. Just be sure to seal it off real good. There are sleeves that you can put around the pipe to stop water from working it's way through the pipe. You can buy them, or build your own. Then the water can flow through the pipe while you are building the dam, and drive from side to side over it too. When the dam is done, you can put in a stand pipe to handle the normal water flow from the creek. If I was doing it, that's probably how I'd approach it.
Good luck,
Eddie
