Complex sentence: Flowing northward between the Oregon Coast Range and the Cascade Range, the river and its tributaries form the Willamette Valley, a basin that contains two-thirds of Oregon's population, including the state capital, Salem, and the state's largest city, Portland.
Split into simpler sentences: Flowing northward between the Oregon Coast Range and the Cascade Range the river and its tributaries form the Willamette Valley. This basin contains two-thirds of Oregon's population which includes the state capital, Salem, and the state's largest city, Portland.