The current Portland Basin (Biogeographical Context Map) marks the northern terminus of the Willamette River lowland portion of the greater Puget-Willamette Trough of the Cascadia Subduction System (Evarts et al. 2009). While the Puget-Willamette Trough is bound by the Coast and Cascade ranges regionally, the Portland Basin is locally bound by the Portland Hills (Columbia River Basalt Group) to the southwest and older Paleogene volcanic rocks to the east. The Portland Basin is roughly 40 miles long, 20 miles wide, and is oriented with its long axis to the northwest.
Over the last 20 million years, the region around the Portland Basin has been subjected to local tectonic and volcanic activity as well as regional flood-basalt flows and extraordinary glacial-outburst floods. These events are captured in the sediment record because the Portland Basin is bisected by the lower reaches of the Columbia River. The majority of the basin is filled with as much as 1,800 feet of Columbia River sediments, carried from the east, ranging in age from Miocene period to present. The present surface is underlain by as much as 400 feet of silt, sand, and gravel deposited by the late Pleistocene cataclysmic Missoula Floods.
We are located in the northern reach of the Portland Basin, and the formation of the Portland Basin was influenced during times of advanced glaciations by the deposition of voluminous glacial outwash events from the Lewis River (Evarts et al. 2009). Later periods of Mount St. Helens eruptions produced lahars and sediment that came down the Lewis River into the northern part of the Portland Basin, where extensive bottomlands were formed at the mouth of the Lewis River (present day Woodland, Woodland Bottoms, Plas Newydd Farm, and Sauvie Island) (Evarts et al. 2009).
We are located in the area that Lewis and Clark mapped and described as Wapato Valley (the lower Columbia River valley, including the Willamette River valley up to about modern Oregon City falls, between the Coast and Cascade mountain ranges) (Coues 1893, Moulton 1983) (Biogeographic Context Map). The name Wapato Valley was given during their 1805–1806 expedition because of the dominance of wapato in the cultural and ecological landscape (Deur and Turner 2005, Coues 1893, Moulton 1983, Burroughs 1995). Portland Basin sedimentation patterns created the ideal hydrogeomorphic floodplain conditions to support vast wapato communities, and expanses of wapato-filled wetlands anchored Chinookan village sites, provided food security, were used as exchange networks for trade commodity, and were used in the development of specialized tools (Coues 1893, Darby 1996) all throughout the Portland Basin. The following passage from Keeping it Living (Deur and Turner 2005) describes the ecological and economic sustainability vision of the Wapato Valley, which closely match the Plas Newydd management philosophy and the overarching goals of our programs:
“In the late eighteenth and early nineteenth centuries, Wapato Valley was an ecologically complex and productive environment that provided the region’s human inhabitants with numerous types of food, with many resources (most notably salmon runs) varying considerably over time and space. The Lower Columbia region fits the model put forward by D. R. Harris (1977) of an emergent stable agricultural system, characterized by an ecosystem with high species and pattern diversity, intensive management of some resources within the ecosystem, and plant ecology that was conducive to intensification.”Back