I'm no expert, but to me it looks like a pretty large lake and not a lot of run-off. You'd better check with a professional before you start building a dam. Ideally, you want to connect two rises in elevations (hills) with your dam. (Think of it as a "U" shape. The sides are the hills and the bottom is the dam.) I don't see two hills to be connected, so it must be more of an embankment pond. Meaning, you'll have a circle dam.
Also, if you're expecting run-off from the west, the road you've drawn will prevent most of the water from flowing into the pond. The road needs to be slightly raised to avoid wash out and in doing so will redirect water AWAY from your pond.
