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I am sorry to hear about this situation. This sounds like a a matter rooted in water rights.
There are three major doctrines that apply in such cases:
COMMON ENEMY RULE
Each landowner has the right to alter the drainage pattern of his land without regard for the effects on neighboring parcels, as long as that water flows to where it otherwise would have naturally flowed.
CIVIL LAW RULE
The owner of a lower parcel of land must accept the natural drainage from those parcels above his, and cannot alter the drainage pattern of his own land to increase the drainage flow onto parcels lower than his own
REASONABLE USE RULE
The landowner can make "reasonable" alteration to the drainage pattern of his parcel, with liability only occurring when the alteration causes "unreasonable" harm toward neighboring parcels.
Oregon generally subscribes to the civil rule and the reasonable use rule, depending on the situation. Anyone who is in breach of this rule may be sued for negligence, civil trespass, and nuisance.
Few attorneys practice exclusively water rights. As such, an attorney in the following areas of law should suffice:
REAL ESTATE (with a focus on development, possibly)
May I recommend the Oregon Bar referral program - HERE. The attorneys are vetted and qualified. You should be able to find an attorney you are confident with and whom you can trust, and who is available ASAP.
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