I had a suspicion, so I called my township and they confirmed it.
“In the case where water runoff from one property is aversely affecting an approved, physical structure on another, it is the responsibility of the first party to correct the problem. In the case of a new development, the responsibility falls on the developer unless it can be proven that alterations made by the new home owner were the cause, in which case that new homeowner is responsible.”
“In the case of established developments where alterations by a homeowner have altered the drainage such that it adversely affects the physical structure of another property, that homeowner is responsible for correcting the offense,
“In the case where a property has an existing stream, flood plane, retention, or detention basin, the homeowner usually is not responsible if such things cause to adversely affect other physical structures of other properties, unless said homeowner has made modifications which alter such items.”
“In cases where changes due to acts of god, environment, etc., cause the problem, the homeowner is not responsible.”
It sounds like you may have a case. Of course, it would have to be proven that it is, indeed, a direct result of your neighbor’s yard modifications. You could call your municipality since they could quickly assess the situation and be able to force the offending homeowner to make the change.
It’s a question of being neighborly, though. Long after this gets resolved, you will have to live next to this person. I would suggest you try every avenue to resolve it on a personal level before going the legal route.