[Protagoras]

If the landscaping causes a gully that erodes the ground around a fence post -- so be it.

That is incorrect in as far as my county in Illinois is concerned. I'm willing to bet it is widespread elsewhere in the zoning laws. You cannot make adjustments to your property which have a new impact on the ajoining property with regard to water flow. That makes your water issue a good example. You cannot make an alteration which makes water flow onto the ajoining property in a different way or in increased volume.

[bvw]

And does the fence act as a dam then? It might. Pictures don't show it.

[bvw]

The water issue is true also in PA, I've heard it brought up before the local zoning boards. Still, it might be that a gully is usually a natural thing, and as long as it forms incidentally to other work and is not of great magnitude, it would be hard to press a case against the homeowner it forms on.