Oregon trail sources:

The Oregon Trail: An American Saga Paperback – 8 Dec 2005
by David Dary

The Oregon Trail: A Primary Source History of the Route to the American West
By Steven P. Olson

https://www.nps.gov/oreg/planyourvisit/maps.htm

https://oregonencyclopedia.org/articles/oregon_trail/#.XZec-EZKiUk

https://www.octa-trails.org/articles/origins-of-the-oregon-trail/