Willamette Valley Ecoregion: Chapter 3 in Status and trends of land change in the Western United States--1973 to 2000

The Willamette Valley Ecoregion (as defined by Omernik, 1987; U.S. Environmental Protection Agency, 1997) covers approximately 14,458 km² (5,582 mi²), making it one of the smallest ecoregions in the conterminous United States. The long, alluvial Willamette Valley, which stretches north to south more than 193 km and ranges from 32 to 64 km wide, is nestled between the sedimentary and metamorphic Coast Ranges (Coast Range Ecoregion) to the west and the basaltic Cascade Range (Cascades Ecoregion) to the east (fig. 1). The Lewis and Columbia Rivers converge at the ecoregion’s northern boundary in Washington state; however, the majority of the ecoregion falls within northwestern Oregon. Interstate 5 runs the length of the valley to its southern boundary with the Klamath Mountains Ecoregion. Topography here is relatively flat, with elevations ranging from sea level to 122 m. This even terrain, coupled with mild, wet winters, warm, dry summers, and nutrient-rich soil, makes the Willamette Valley the most important agricultural region in Oregon. Population centers are concentrated along the valley floor. According to estimates from the Oregon Department of Fish and Wildlife (2006), over 2.3 million people lived in Willamette Valley in 2000. Portland, Oregon, is the largest city, with 529,121 residents (U.S. Census Bureau, 2000). Other sizable cities include Eugene, Oregon; Salem (Oregon’s state capital); and Vancouver, Washington. Despite the large urban areas dotting the length of the Willamette Valley Ecoregion, agriculture and forestry products are its economic foundation (figs. 2,3). The valley is a major producer of grass seed, ornamental plants, fruits, nuts, vegetables, and grains, as well as poultry, beef, and dairy products. The forestry and logging industries also are primary employers of the valley’s rural residents (Rooney, 2008). These activities have affected the watershed significantly, with forestry and agricultural runoff contributing to river sedimentation and decreased water quality in the Willamette River and its tributary streams (Oregon Department of Fish and Wildlife, 2006). Recent years have seen a marked decline in forest health related to the increased frequency of multiyear droughts. Insect damage and other diseases also are present; however, drought- related water stress is the primary factor in coniferous-tree mortality (Oregon Department of Forestry, 2008). Trees most at risk include Douglas-fir (Pseudotsuga menziesii), grand fir (Abies grandis), and western red cedar (Thuja plicata). Overstocking by timber companies and planting on sites with poor conditions increase susceptibility. Over time, these problems may lead to changes in planting practices and the use of more drought-tolerant species such as ponderosa pine (Pinus ponderosa).