The rugged, volcanic Cascade Range stretches from southern British Columbia to northern California; it is bisected by the Columbia River, which forms most of the Washington-Oregon border. When enumerating the primary rivers that drain the Cascades, it is helpful to use major landmarks of the range from north to south: Snoqualmie Pass (on Interstate 90), Mt. Rainier, Mt. St. Helens and Mt. Adams, the Columbia River, Crater Lake and Mt. Shasta.
The Northern Cascades (north of Snoqualmie Pass) are drained on the west by (north to south), the Skagit, Stilaguamish, Snohomish and Snoqualmie Rivers and their numerous tributaries; their east slope is drained by tributaries of the Columbia: the Okenagan, Methow, Stehekin, Entiat, Swakane and Wenatchee Rivers (north to south). The west flank of the Cascades between Snoqualmie Pass and Mt. Rainier is drained by the Cedar, Green, White, Carbon, Puyallup and Nisqually Rivers (north to south) while the east side of that segment is drained by the watershed of the Yakima River, which enters the Columbia at Richland, Washington. The Cowlitz River drains the west slope of the Cascades between Mt. Rainier and Mt. St. Helens and the Lewis River drains the region between the latter mountain and the Columbia River; the Klickitat, White Salmon and Wind Rivers drain the east side of the range from Mt. Adams to the Columbia River (entering the latter, in order, from east to west)
In Oregon, the Williamette and Deschutes Rivers parallel the west and east flanks of the Cascades, respectively, flowing northward to the Columbia from the area just north of Crater Lake and fed by numerous tributaries along the way. The Umpqua and Rogue Rivers drain the west side of the Crater Lake region (north to south) while the Klamath River drains its eastern and southern flanks, flowing southwest into Northern California and thence to the Pacific; enroute, the latter river receives flow from the Shasta River, which angles northwest from its namesake mountain. Finally, the southern and eastern flanks of Mt. Shasta and the western and southern slopes of Lassen Peak are drained by the Sacramento River and its upper tributaries.
