With a variable topography, the Portland area ranges from river valley floors to terraces reaching elevations of . The Willamette Valley is separated within by hills reaching heights more than , and it is also physically separated from the lower Columbia River valley. The Columbia River flows west from the eastern Portland region, merging with the Willamette near Portland before moving north. Tributaries for the Willamette include the Pudding, Molalla, Tualatin, Abernethy, and Clackamas Rivers, while the Washougal and Sandy Rivers mark notable tributaries for the Columbia River. The Columbia River has significantly shaped the geology of the area.