The valley is synonymous with the cultural and political heart of Oregon and is home to approximately 70 percent of its population[1] including the five largest cities in the state: Portland, Eugene, Salem, Gresham, and Hillsboro.

[2] 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.

Today, the valley is often considered synonymous with "Oregon Wine Country", as it contains more than 19,000 acres (7,700 ha) of vineyards and 500+ wineries.

[3] 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 300 to 400 feet (91 to 122 m) above current sea level.

[4] 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 of 1903.

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.

Conversely, at the northern end of the valley, Portland is 50 feet (15 m) above sea level and receives only 36 inches (910 mm) per year.

[9][10] 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.

The floodwaters carried rich volcanic and glacial soil from Eastern Washington, which was deposited across the valley floor when the waters subsided.

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 15,000 acres (60 km2) with steep terrain and certain species.

)[22] 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.

This landscape was maintained by the Native American inhabitants of the valley who set frequent fires that 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.

North Pacific Oak Woodland is a major forest alliance, extending through the Willamette Valley and southward to the Klamath Range of Northern California.

[30] Salmon, deer, and camas bulbs[31] have provided primary food sources for the valley's first residents who used fire to encourage persistence of oak savanna.

[32] Kalapuya, Chinook, and Molala peoples of the Willamette Valley currently are included among the confederated tribes that make up the Grand Ronde and Siletz Nations.

Today, the people of the Confederated Tribes of Grand Ronde refer to their removal from the Willamette Valley as a Trail of Tears.

[35][36][37][38] After reports of the Lewis and Clark Expedition were published in about 1807, a small and steadily increasing stream of isolated pioneer groups began settling in the valley and improving routes from the east set up by fur traders and mountain men.

The Hudson's Bay Company controlled the fur trade in the valley and the rest of Oregon Country in the 1820s and 1830s from its Columbia District headquarters at Fort Vancouver.