Oregon Revised Statutes Counties and County Officers § 201.310

The boundary of Union County is as follows: Commencing at a point where the forty-sixth parallel of latitude crosses the summit of the Blue Mountains; thence east along such line to its intersection with Snake River; thence up the middle of the channel of the river to the mouth of the Powder River; thence up the middle channel of the river to the mouth of the north fork of the same; thence up the main channel of the North Powder River to its source; thence west to a point intersecting the east boundary line of Umatilla County; thence northerly along such line to the place of beginning.