The Willamette Valley ( ) is a valley in Oregon, in the Pacific Northwest region of the United States. The Willamette River flows the entire length of the valley and is surrounded by mountains on three sides: the Cascade Range to the east, the Oregon Coast Range to the west, and the Calapooya Mountains to the south. The valley is synonymous with the cultural and political heart of Oregon and is home to approximately 70 percent of its populationLoy, et al., p. 35 including the five largest cities in the state: Portland, Eugene, Salem, Gresham, and Hillsboro.
The valley's numerous waterways, particularly the Willamette River, are vital to the economy of Oregon, as they continuously deposit highly fertile alluvial soils across its broad, flat plain. A massively productive agricultural area, the valley was widely publicized in the 1820s as a "promised land of flowing milk and honey". Throughout the 19th century, it was the destination of choice for the ox-drawn wagon trains of emigrants who made the perilous journey along the Oregon Trail.
Today, the valley is often considered synonymous with "Oregon Wine Country", as it contains more than of vineyards and 500+ wineries.
Geology Much of the Willamette's fertility is derived from a series of massive ice-age floods that came from Lake Missoula in Montana and scoured across Eastern Washington, sweeping its topsoil down the Columbia River Gorge. When floodwaters met log- and ice-jams at Kalama in southwest Washington, the water caused a backup that filled the entire Willamette Valley to a depth of above current sea level.Cataclysms on the Columbia, by John Elliott Allen and Marjorie Burns with Sam C. Sargent, 1986. Pages 175–189. Some geologists suggest that the valley flooded in this manner multiple times during the last ice age.Geology of Oregon, by Elizabeth L. Orr, William N. Orr and Ewart M. Baldwin, 1964. Pages 211–214. (If floodwaters of that magnitude covered Portland (elevation ) in 2010, only the tops of the West Hills, Mount Tabor, Rocky Butte, Kelley Butte and Mount Scott would be visible, as would only some of the city's tallest skyscrapers.) Elevations for other cities in the valley are Newberg at , Oregon City at , McMinnville at , Salem at , Corvallis at , and Eugene at . The lake gradually drained away, leaving layered sedimentary soils on the valley floor to a height of about above current sea level throughout the Tualatin, Yamhill, and Willamette valleys.
Geologists have come to refer to the resulting lake as Lake Allison, named for Oregon State University geologist Ira S. Allison, who first described Willamette Silt soil in 1953 and noted its similarity to soils on the floor of former Lake Lewis in Eastern Washington. Allison is also known for his work in the 1930s documenting the hundreds of non-native boulders (called erratics) washed down by the floods, rafted on icebergs and deposited on the valley bottom and in a ring around the lower hills surrounding the Willamette Valley. One of the most prominent of these is the Bellevue Erratic, just off Oregon Route 18 west of McMinnville.
It is also believed that the Willamette Meteorite was rafted by flood and ice to the location near West Linn where it was discovered and venerated by the Clackamas people up until its discovery by settlers in 1902, prompting a lawsuit regarding its ownership that reached the Oregon Supreme Court in July 1903.
Geography The valley may be loosely defined as the broad plain of the Willamette, bounded on the west by the Oregon Coast Range and on the east by the Cascade Range. It is bounded on the south by the Calapooya Mountains, which separate the headwaters of the Willamette from the Umpqua River valley about south of Hidden Valley. From an eccological perspective, it continues a little north of the Columbia River into southwestern Washington, up to the Lewis River, but the Willamette River terminates where it flows into the Columbia and gererally this is considered the northern boarder of the basin. Interstate 5 (I-5) runs the length of the valley, linking its major communities.
Because of differing cultural and political interests, the Portland metropolitan area and Tualatin River valley are sometimes not included in the local use of the term, though both are located within the geographical valley. Additionally, the east slopes of the Coast Ranges and the west slopes of the Cascade Range from Oakridge to Detroit Lake can be considered part of the Willamette Valley in a cultural sense, despite being mountainous areas.
Cities in the valley include, from south to north, Cottage Grove, Eugene and Springfield (the two cities form a single populated area, separated in places only by I-5 and/or the McKenzie River), Corvallis, Albany, Dallas, Salem, Keizer, McMinnville, and Hillsboro. Parts of other counties lie within the valley (from south to north): Douglas, Lane, Linn, Benton, Polk, Marion, Yamhill, and Washington counties. Sometimes the area around Albany and Corvallis and surrounding Benton and Linn counties is referred to locally as the Mid-Valley. Marion, Polk, and other counties are sometimes included in the definition of the Mid-Valley.
Climate
The climate of the Willamette Valley is a mix of Mediterranean (Köppen Csb) and oceanic (Köppen Cfb) influences. The Köppen climate classification system considers it Mediterranean, but compared to a true Mediterranean climate it is cooler and moister, with a longer rainy season. The main climatic features are moderate temperatures and frequent cloudiness and rains, except in summer when the northward expansion of the North Pacific High creates generally sunny and warm weather.
Winters are consistently wet and cloudy, and often foggy, but quite mild. Although night frosts are common, the temperature almost always rises above the freezing point in the daytime. Snow occurs on occasion, but accumulations are normally light, and in some winters no snow whatsoever falls. Very cold temperatures are atypical; the temperature very rarely falls below , and readings of or lower occur only about once every 25 years. Summers are characterized by warm, sunny afternoons with little or no humidity, and cool evenings. Sometimes, heat waves can occur, with temperatures rising above and occasionally even reaching , but the nights usually bring relief. More recently, there have been heat waves in the area that reach higher than .
Precipitation varies considerably across the valley and is closely correlated with elevation. Annual totals range from at the lowest elevations to more than in the foothills. Eugene, at the southern end of the valley, is above sea level and receives per year. Conversely, at the northern end of the valley, Portland is above sea level and receives only per year. Most rainfall occurs from October to May, and it tends to be heaviest between November and January, when disturbances coming from the Pacific Ocean are at their most intense. Growing seasons are long, averaging 150 to 180 days per year in the lowlands to about 110 to 130 days at elevations above .
Severe storms of any kind are rare, although snow and ice storms can sometimes occur when surface low pressure systems move south along the coast, inducing offshore flow which advects cold air from the Columbia Basin westward through the Columbia River Gorge, filling the valley to the north of the surface low track. Weather systems sometimes bring high winds to the northern region of the valley. Average cloud cover can exceed 70 percent in winter but drops to less than 15 percent in summer. Tornadoes are rare, but do happen a few times a year with minimal damage.
Agriculture
The agricultural richness of the valley is partly due to the Missoula Floods that inundated the valley approximately 40 times between 15,000 and 13,000 years ago at the end of the last ice age. The floods were caused by the periodic rupturing of the ice dam of Glacial Lake Missoula, the waters of which swept down the Columbia River and flooded the Willamette Valley as far south as Eugene. The floodwaters carried rich volcanic and glacial soil from Eastern Washington, which was deposited across the valley floor when the waters subsided. The soil in the Willamette Valley is about deep in some areas.
In the cool moist climate of the Willamette valley, over 170 different crop and livestock items are produced, including grass and legume seeds, tree fruits and nuts, wine grapes, berries, vegetables, nursery, Christmas trees, and field crops such as wheat, oats, mint and hops, hay, livestock, and poultry. The valley produces most of the cool-season forage and turf grass, Christmas trees, and hazelnuts sold in North America. It is also noted for its hops, which are widely used in craft beer and microbreweries throughout the US.
In recent decades, the valley has also become a major wine producer, with multiple American Viticultural Areas of its own. With a cooler climate than California, the gently rolling hills surrounding the Willamette are home to some of the best (and most expensive) Pinot noir in the world, as well as a high-quality Pinot gris. Although this distinction is not officially recognized, many wine connoisseurs further divide the Willamette Valley into northern and southern regions, the dividing line being the approximate latitude of Salem (approximately 45° north latitude). Not all portions of the Willamette Valley are suitable for vineyards, however, and the largest concentration of wineries is found west of the Willamette River, on the leeward slopes of the Coast Range, or among the numerous river and stream valleys created by Willamette River tributaries. By far, the largest concentration of wineries is in Yamhill County.
Grass farmers have been burning fields, as part of their production, since the 1940s. The smoke is often irritating to residents; in 1988 it caused a 23-car pileup on I-5. Over the years, several pieces of legislation have limited the amount of burning permitted. With the passage of a bill championed by legislator Paul Holvey in the 2009 session, burning has been banned since the summer of 2010, with the exception of an area of about with steep terrain and certain species. (At its peak in the 1980s, about were burned each year.)
The marionberry, a cross between Chehalem and Olallie blackberries, was bred at Oregon State University as part of a berry-developing partnership with the US Department of Agriculture that dates back to the early 1900s. It is named for Marion County in the Willamette Valley, where most of the field trials took place. When the berry was introduced in the 1950s, it was widely hailed as the most delicious blackberry commercial cultivar around. Even today, people rave over its tart-yet-sweet flavor, described as a cross between raspberries and blackberries. (Though there is some raspberry in its DNA, the red fruits are more like a genetic great-great-grandparent to the marionberry.)
Ecology
The Willamette Valley is prone to periodic floods. Notable floods include the Great Flood of 1862, events in 1899, the Christmas flood of 1964, and the Willamette Valley flood of 1996. Part of its floodplain is a National Natural Landmark called the Willamette Floodplain.