You definately want to understand the law before digging. In Indiana, I would start with the DNR because they seem to be most interested in promoting wildlife rather than acting as policemen. In Virginia, it looks like your agency is DGIF (see link below). They could probably point you in the right direction. Flood plains are handled differently in different places. I just had a wetland mitigated in a flood plain on my place, but it was a previously cultivated field.

http://www.dgif.virginia.gov/about/