The map below shows the extent of Oregon Territory as it existed until 1846 (the map itself shows the Territory in 1838). The territory was bounded on the south by California (at the 42nd parallel) and bounded on the north by the 54th parallel (which was also the southern border of Alaska). On the east, the territory was bounded by the Rocky Mountains. The extent of Lewis County at its creation in 1845 is outlined in red. Vancouver County (later Clark County) formed the remainder of the territory north of the Columbia river.
(Map courtesy of John Planinshek and posted on the Lewis County Genealogical Society website)