All this Salmon river research has led me to the Oregon State Archives. There they have the records of the Oregon State Highway Commission. Commission records go back to about 1911 when the commission was formed and there is a set of logbooks of commission discussions and well as a set of maps (some maps appear missing). I spend about two days at the archives and found in the journals the progress of the conversion of the Salmon River Wagon Road to the State highway from its earliest discussion in 1917 until at least 1933 when they were installing signage. The project crossed through numerous jurisdictions, Polk County, Tillamook county, Lincoln County, with perhaps a small piece in Yamhill County. As well there had been two Indian reservations there, Grand Ronde and Siletz. The land in the footprint of the road was by-and-large sold from both reservations by 1907, but the federal government still had interests in the section of the road that ran through Grand Ronde on the way east to Yamhill County and Marion County.
In the collection mentioned, I found the original blueprints of the Tillamook County section of the Salmon River Highway. On the map pages was noted the original wagon road and the proposed new highway route. There is also noted the homesteads, houses and present owners of each of the parcels of land along the route. The rivers are also noted, what appears to be the headwaters of the Little Nestucca, The Little Salmon River, several creeks, a Sulpher Springs and the main stem of the Salmon River. The blueprints also have a section depicting elevations of the route. There is enough detail that if we wanted to we could find the original trail based on a number of junctions that still exist today.
Note that the highway construction did not just change the route of the road, but on the maps there are numerous notations about changing the route of the river too. We know that bridges are hugely expensive to build, so when possible the engineers would change the actual course of the rivers to avoid bridges. The Sulpher Springs section of the river would have caused the engineers to build two bridges but when they placed a rock crib and redirected the river, this eliminated the need for the two bridges. There may instead be small culverts to help water flow of the resulting creeks. The section of the river cut off appears to have been a wide section perhaps a slow section of the river with a marsh or pond.
Salmon River Highway is one of the primary routes in Oregon today. The original Elk Trail that became a wagon road is an important route to the Oregon Coast, one of the most well traveled and picturesque of the routes to the coast.