Complex sentence: These three Southern Oregon rivers drain mountains south of the Willamette Valley; the Willamette River and its tributaries drain north along the Willamette Valley into the Columbia River, which starts in British Columbia rather than Oregon.
Split into simpler sentences: These three Southern Oregon rivers drain mountains south of the Willamette Valley. The Willamette River and its tributaries drain north along the Willamette Valley into the Columbia River, which starts in British Columbia rather than Oregon.