Soil development, texture, and drainage are specific to geomorphic surfaces expressed in the valley. In the Puget lowlands and foothills bounding the lowlands, soils have formed in and on drift deposited by continental glaciation. Soils are related to a complex mosaic of deposits resulting from glacial processes. Potential Natural Vegetation. Long-term human settlement has had pronounced influence on this area.
More recent investigations suggest somewhat different interpretations of potential vegetation.
The Willamette Valley portion is a mixture of Douglas-fir and White Oak series with local areas of Western Hemlock and Western Red Cedar series on more moist sites. A similar pattern occurs in the Puget Trough portion, but with a greater abundance of Western Hemlock and Western Red Cedar series and less amount of White Oak and Douglas-fir series. Riparian areas, with aquic conditions, include Cottonwood, Willow, Ash, and Alder series. Bigleaf maple occupies mixed sites. Tufted hairgrass and shrub thickets are dominant in wetlands.