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Portland/Vancouver Basin

↑About the Portland/Vancouver Basin

The Portland/Vancouver Basin is a basin at the north end of the Willamette Valley, containing the centers of both Portland, OR and Vancouver, WA (a small city distinct from, and nowhere near Vancouver, BC) but not containing their entire metropolitan area. This region is a fault block basin, meaning it formed when a very large block of rock dropped downward relative to the surrounding mountains, as the earth's crust was being pulled apart. As such, the borders of this region are well-defined and rather linear and abrupt.

This region consists of rolling terraces and floodplains, with numerous ponds, wetlands, and oxbow lakes. Elevations range from 0 to 300 feet. Although the terrain is mostly flat, there are some isolated buttes, becoming more frequent towards the south, and some cliffs along the north bank of the Columbia river, particularly in the east. The region is underlain by a mix of glacial and stream deposits. The soils are quite variable across the region, and range from fertile and rich in organic matter in places, to more nutrient-poor in others.

This region has a temperate oceanic climate with a strong seasonality of precipitation, with dry and sunny summers, but very wet and cloudy winters. Unlike areas farther south in the Willamette Valley, this region can be affected by winds blowing from the east along the Columbia river gorge.

Before European settlement, this area was maintained as prairies by Native Americans who practiced controlled burns. However, the rainfall and soil type probably would have supported closed-canopy forests were it not for this intervention. Dominant trees at the time of settlement were oregon white oak (Quercus garryana), douglas-fir (Pseudotsuga menziesii), oregon ash (Fraxinus latifolia), alder, and western redcedar (Thuja plicata).

Nowadays, this region is heavily urbanized, but there is also significant agriculture, including both pastureland and nursery crops. This region includes the bulk of Portland, but the city limits extend both farther west and farther south, outside this region, and the metro area extends much farther west and significantly farther east and south as well. There is little forest cover remaining; what little protected land exists is mostly either on the isolated buttes and cliffs, and some areas of protected wetlands in the floodplain of the Columbia river, the largest being Ridgefield National Wildlife Refuge.

This region is surrounded to the northeast, east, and southeast by the Valley Foothills. At the south there is a small border with the Prairie Terraces. To the southwest and west it directly borders the more rugged, mountainous Volcanics of the Coast Range. Interspersed with this area, there are a few borders to the west with the Willapa Hills.

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↑References

1. Pater, D.E., Bryce, S.A., Thorson, T.D., Kagan, J., Chappell, C., Omernik, J.M. "Ecoregions of Western Washington and Oregon (Poster)", U.S. Dept., of the Interior, U.S. Geological Survey, Reston, VA (0000) Web.