Let me second Bob in saying...hire a professional engineer. It is important that the engineer have a hydraulics and hydrology background. If you hire an engineer that does not specialize in H&H, it will cost you more $$$ because they won't be as efficient.

The H&H engineer will also be able to address the 2nd suggestion Bob mentioned. These are the ways you can cut your costs and do the job right at the same time, the first time. Hire an engineer who is knowledgable about ponds, has good contacts with contractors and can supervise your project to make sure it's done right.

Just my 2 cents.