In remembrance of the Kalapuyan and Clackamas (lower Columbia Chinook) [I]ndians who lived and died here, and in honor of those who still live here; please stop saying “no one lived here.” Please stop saying that Willamette means “the valley of sickness and death.” Please know that if the natives later refered to this valley as one of “sickness and death,” it came from the biological genicide inflicted on the natives by this [i.e., Euro American] civilization. Please go to the library, or better yet find a living native, and learn the real history of this place.
Urban Scout does a great job of sleuthing that includes critiquing "explanations" of the name that do not cite a source and then referencing scholarly works on the topic, particularly Henry Zenk's essay in the Handbook of North American Indians, Vol. 7: Northwest Coast (1990). To add a little to this . . .
Lewis and Clark identified the river we now know as the Willamette as the Multnomah in their journal entries of early April 1806, though the Chinookan word that served as their source refers only to a village on Sauvie Island (/máånumaå/, “those towards the water”).
Henry Zenk finds that trappers and Indians extended the name “Willamette” (also spelled wálamt, wallamat, wolamat, and wolamut) to the entire river and watershed by the time the Pacific Fur Company founded Fort Astoria in 1811. Zenk posits that this is most most likely by way of the lingua franca of the Columbia basin, Chinuk Wawa. Previously, the name appears to have only applied to the name of a Chinook village on the west side of Willamette Falls that had long served as an important center of trading, at least until the large-scale depopulations from malaria outbreaks between 1830 and 1834. We have no definitive record of what the name “Willamette” meant to the Chinooks.
 I hadn't heard this explanation before reading this Urban Scout's blog.
 Henry Zenk, "Notes on Native American Place-names of the Willamette Valley Region," Oregon Historical Quarterly 109: 1 (Spring 2008), 25-26. See entries for April 2- 6, 1806, in Gary E. Moulton, ed., The Journals of the Lewis & Clark Expedition (Lincoln, University of Nebraska Press, 1983-2001), http://lewisandclarkjournals.unl.edu, accessed May 3, 2010.