Re the origin of the term "Valley of Sickness" as a descript
or of this region, the article linked below is a rich source of information on how this phrase may have come to be.
Note that I say
may because there is no definitive reason for the name, just theories, the article says.
The theories range from early Indians settling in a mosquito-infested place and catching diseases, to, surprisingly, the opposing theory that the valley was in fact a beautiful place where people went to be
cured of sickness!
It seemed to me from reading the article that the most historically supportable theory involves plague-like conditions imported into the valley from the outside:
"(The) Kalapuyan (Indian) populations suffered catastrophic declines during early historical times, the most dramatic single decrease probably occurring during 1830 to 1833, when malaria swept the Willamette and lower Columbia areas. There is no reliable data on how large Kalapuyan populations were before this disastrous event. (Handbook of North American Indians, Volume 7, pg. 551)"And
"The settlers, missionaries, and explorers of the period 1830-1855 give an account of the Chinookans that reflects the population decline experienced by these people from smallpox, measles, malaria, and other diseases. Most of the earlier sites were abandoned, or with reduced consolidated populations, particularly Multnomah and Clackamas Chinookians, who were thoroughly ravaged… (Handbook of North American Indians, Volume 7, pg. 535)"But the other theories remain plausible until more definitive research decides the matter.
And here's a little extra, the possible meaning of "Willamette":
"Willamette is a Clackamas Chinook word. “Mette” is the french spelling that was changed in the 1800’s from “met” which was a poor way of spelling the Chinookan word “maɬ.” Maɬ is the chinookan word for water or river. The barred L which sits at the end of the word is not a letter in the English alphabet and so it was never recorded as such, except by linguists. You can see it tagged onto the end of many river names: Clackamas, Multnomah, Willamette, Klamath, and others."Hence, "Willamette" means "the water or river of Willa," although the article says the meaning of "Willa" remains unclear.
https://www.petermichaelbauer.com/willamette-the-valley-of-an-8000-year-old-culture/#:~:text=in%20the%20fertile%20willamette%20valley%2c%20which%20indians%20named,broken%20covenants%20and%20treaties%20were%20a%20severe%20problem.Probably as much as we want to know about
the subject.