You have not supplied enough information. Who owns the ditch? Who owns it at the point of the dam? If the trench that drains the ditch is along the property line can you direct your pumped water there instead? If the ditch is owned (or controlled through an easement) by a local government unit you can probably complain to them about the dam.
Shell Bleiweiss
http://www.shell-bleiweiss.com
