Allow me to cite a recent example from here in PA: PennDOT had blocked off one of its own cross-drain culverts for whatever reason decades ago. The landowner on the downgradient side built a golf course. Last year PennDOT decided to reopen its culvert, arguing (and proving) that it was not increasing runoff to this point, but merely utilizing an existing drainageway. No easement was ever established and none is currently proposed. The culvert was opened and remains opened and the golf course now has considerably more water traps than they had designed. The golf course owner is free to convey the water anyway it chooses, but it cannot force the obstruction of a naturally occurring watercourse.
It would be interesting to see the stipulations of this curious agreement johnhan76 cites...how much water is defined as required to be held back? What happens with any in excess of that? In other words, is the berm (which is really just a detention (or more accurately a retention) basin) defined by a release rate? If this rate is zero, what are the previously stipulated and mutually agreed upon consequences of violation...say during the 500-year storm, an event which all but the most stubborn stormwater engineers and professors admit has not yet been quantified, but only arrived at through extrapolation? Or, is the berm/detention basin defined by a) height of embankment, b) volume c) area, d) perimeter? Can any of these ever be altered, or are they all defined? Just saying someone agreed to build a berm is flimsy at best, and probably legally meaningless.
I find it hard to comprehend a legally-binding agreement which could prohibit naturally occurring runoff from being allowed to continue along its course. Of course, the berm is probably now concentrating other runoff to the right outfield, which I will assume is (just barely) the lowpoint. So the berm would have to be entirely removed to truly restore the natural drainage patterns.
To keep peace, the county in your scenario should probably just buy an easement off the adjoiner. Perhaps they could agree to pipe it to the ditch. But I am certain such a dubious agreement would be childsplay for a decent attorney to pick to shreds, if your client is up for a long, costly battle which will certainly create bad blood...and require lots of engineering calcs and testimony!!!