From the time of the formation of the Oregon Territory, thru its admission to the United States in 1859 until the counties assumed their present form in 1908, there were numerous boundary changes. The original Oregon Territory included land that later became part of Montana & Wyoming as well as all of present day Washington and Idaho. The original counties, Champoeg, Clackamas, Washington & Yamhill were formed in 1843. From these four, came all the other 36 counties. To see the various county boundary changes, click here.