The Willamette Valley ecoregion is a Level III ecoregion designated by the Environmental Protection Agency, and includes the Willamette Valley of western Oregon and adjacent foothills along with a small portion across the Columbia River in the Vancouver area of southern Washington (EPA 1996, ODFW 2006). It is one of the smallest ecoregions in the United States with an area of approximately 5,800 square miles; the Oregon portion encompasses 5,308 square miles of land between the Coast Range and the Cascades Mountains (ODFW 2006). Twenty to 40 miles wide and 120 miles long, elevations range from 780 feet at the southern end south of Eugene to near sea-level at Portland (ODFW 2006, USGS 2012).
The valley is a long, level alluvial plain with scattered groups of low basalt hills, with a mixture of volcanic and sedimentary soils dominated by silty-loam and clay soils deposited by the Missoula Floods (ODFW 2006, NRCS 2012). Native vegetation in the valley is a mosaic of oak savanna, oak woodland, wet and upland prairies, riparian gallery forests, Douglas-fir forests, and mixed oak/conifer forests. The climate in the Willamette Valley is characterized by mild wet winters and warm dry summers. There are 3,000 miles of creeks, streams, and rivers; the major rivers in the ecoregion are the Willamette, McKenzie, Santiam, Sandy, Mollala, Clackamas, Tualatin, Yamhill, Luckiamute, and Long Tom rivers (ODFW 2006).
Willamette Valley prairies are among the most endangered ecosystems in North America – over ninety percent of upland prairie and oak savanna/woodlands and over 99% of historic wet prairies in the valley have been converted to other uses, primarily urban and agricultural (USFWS 2010, TNC 2008). Fertile soil and abundant rainfall make the valley the most important agricultural region in the state (ODFW 2006, EPA 1996). Seventy percent of the population of Oregon resides in the Willamette Valley, which includes eight of the top ten urban centers in the state (US Census 2010). Due to continued population growth, pressure on valley ecosystems from urban expansion, land-use conversion, and pollution is likely to increase further (ODFW 2006).
Natural Resources Conservation Service, United States Department of Agriculture. Web Soil Survey. Available online at http://websoilsurvey.nrcs.usda.gov/. Accessed January 2, 2013.
Oregon Department of Fish and Wildlife. 2006. Oregon Conservation Strategy. Oregon Department of Fish and Wildlife, Salem, Oregon.
The Nature Conservancy. 2008. Willamette Valley Fact Sheet. http://www.nature.org/ourinitiatives/regions/northamerica/unitedstates/oregon/placesweprotect/wv_fact_sheet.pdf. Accessed December 11, 2012.
U.S. Environmental Protection Agency. 1996. Level III ecoregions of the continental United States (revision of Omernik, 1987): Corvallis, Oregon, U.S. Environmental Protection Agency – National Health and Environmental Effects Research Laboratory Map M-1, various scales.
U.S. Fish and Wildlife Service. 2010. Recovery Plan for the Prairie Species of Western Oregon and Southwestern Washington. Portland, Oregon.
U.S. Geological Survey, Western Geographic Science Center. 2012. Willamette Valley Ecoregion Summary. Menlo Park, CA. http://landcovertrends.usgs.gov/west/eco3Report.html. Accessed January 2, 2013.