Slightly larger than the Willamette Valley for which it is named, the ecoregion contains fluvial terraces and floodplains of the Willamette River system, scattered hills, buttes, and adjacent foothills. It is distinguished from the neighboring Coast Range, Cascades, and Klamath Mountains ecoregions by lower precipitation, lower elevation, less relief, and a different mosaic of vegetation.
Mean annual rainfall is 37 to 60 inches (96 to 152 cm), and summers are generally dry.
Historically, the region was covered by rolling prairies, oak savanna, coniferous forests, extensive wetlands, and deciduous riparian forests. Today, it contains the bulk of Oregon’s population, industry, commerce, and agriculture. Productive soils and a temperate climate make it one of the most important agricultural areas in Oregon.
Less than 1 percent of the Willamette Valley remains as intact habitat due to conversion to agriculture, urbanization, and fire suppression. Practically no prairie remains and the savanna is converting to forest. Most of the riparian areas have been lost, though some remain intact because their propensity for flooding makes them unsuitable for agriculture or development.
The ecoregion covers an area of 14,900 square kilometers (5,800 sq mi), lying mostly in Oregon, with a small portion lying across the Columbia River in southern Washington. The ecoregion lies in the Willamette Valley, which runs from south to north between the Oregon Coast Range to the west and the Cascades Range to the east. The ecoregion is drained mostly by the Willamette River and its tributaries, which flows into the Columbia River straddled by Portland, Oregon.
Historically, the Willamette Valley forests were mostly an oak savanna, tall grasslands with scattered Garry oaks (Quercus garryana), and groves of coast Douglas-fir (Pseudotsuga menziesii subsp. menziesii). The river floodplains contained extensive wetlands, stands of willow, alder, and cottonwood, and gallery forests. The Willamette Valley provides the only habitat for Bradshaw’s lomatium (Lomatium bradshawii), a yellow-flowered member of the parsley family (Noss and Peters 1995). The valley is also the sole wintering area of the dusky Canada goose (Branta canadensis occidentalis) (Bailey 1995).
This landscape was maintained by the Native American inhabitants of the valley who set frequent fires which encouraged the open grasslands and killed young trees. The American settlers of the region, since the 19th century, suppressed fires and converted much of the valley to agriculture, which has caused much of the former grassland and savanna to revert to closed-canopy forest.
Further Reading: Willamette Valley Forests