Oregon Revised Statutes Counties and County Officers § 201.240

(a) The line dividing Marion and Yamhill Counties is a line drawn along the middle of the main channel of the Willamette River where that main channel is located on June 22, 1981. All land on the east side of the line dividing Marion and Yamhill Counties is part of Marion County.

(b) The southern boundary of Marion County and the northern boundary of Linn County commences in the middle in the main channel of the Willamette River opposite the mouth of the Santiam River; thence up the middle of the main channel of the Santiam River as it existed on April 21, 1982, to a point opposite the mouth of the north fork of the river; thence up the middle of the main channel of the north fork as it existed on April 21, 1982, to the mouth of Whitewater Creek; thence up the creek to a point where the creek crosses the section line between sections 21 and 28 in township 10 south, range 7 east, Willamette Meridian; thence due east along the section line projected to the summit of the Cascade Mountains.