The southern boundary of the North Cascades is less definite. For the purposes of this article, it will be taken as U.S. Highway 2, running over Stevens Pass, or equivalently, the Skykomish River, Nason Creek, and the lower Wenatchee River. This roughly follows Beckey's geologic division in Cascade Alpine Guide and the definition used by Peakbagger.com. Sometimes the southern boundary is defined by Snoqualmie Pass and the approximate route of Interstate 90. Sometimes the term North Cascades or northern Cascades is used for the entire range north of the Columbia River.