Cindy,
We are in Texas so things are a little different here as far as rules from the NRCS (SCS) goes. But I would definately go to them first. Their job is to help you as a landowner and most of the guys I have dealt with at the NRCS are friendly and cooperative. They will be able to answer your questions as far as engineering the dam goes and what ramifications your pond and dam may have on people downstream. This is your first and most important step - nothing will matter with the Corps if the NRCS determines that your dam structure is inadequately designed and poses a threat to downstream property owners or structures.

The Corps is a different issue. Navigable waters are very generally defined as anything that shows up as a blue line on a USGS map (they will have this at the NRCS office). If your pond is on a stream that shows up as a blue line on the USGS map it will be determined jurisdictional by the Corps. The problem you are encountering is the fact that you may be held liable for someone else's mistakes. I will tell you in my experience with the Corps (and I have had my share here in Texas) they will not come after you unless someone complains - even then those people will have to complain A LOT in order to get them to act. I am not advocating not following the rules - I am an engineer - I really like rules \:\) .

I just want you to understand that in my experience your level of risk of being investigated by the Corps is slim to none - especially if the pond has been there for two years already. Also, an unwritten rule of thumb is that a condition has to exist for five years before the Corps will consider that condition the "existing condition". You are half-way there. I have taken the Corps wetland certification class and what they will do if they ever come to check your pond out is they will look back at historical aerials and determine how long that pond has been in existence. If they find that it has been there for five years or more - then you are in the clear.

If you have more questions or want to email me directly you can reach me at emily@pondmedics.com.

Hope this helps! Good Luck!
Emily Lewis P.E.