Again, congratulations to Larry. I never would have guessed the POTB and the Nehalem river, but it fits all the clues.

OPB, the Snake river enters the Columbia river just east of Pasco, WA. The UP's mainline over the Blue mtns doesn't get very close to the Snake on the west side but their "Washy" line to Spokane does follow both the Columbia and the Snake north of Hinkle before crossing the Snake and leaving it for good at the impressive Joso bridge near Ayer WA. The Camas Prairie, now Watco's Great Northwest RR, follows the Snake all the way from Ayer WA to Lewiston ID.

I am sure the UP considered a water level grade along the Snake river all the way from southern Idaho to Portland OR; it looks good on paper. However there is a major obstacle along the Idaho-Oregon boarder called "Hells Canyon". This is a very deep and rugged canyon that even today is largely inaccessible. At one time the UP did have a 58 mile branch that went north from a couple of miles east of Huntington OR to Homestead OR, but this branch has been gone since 1958.

The book "Union Pacific Steam - Northwestern District" by James L Ehernberger and Francis G Gschwind, published 1975 by E G Publications, P O Box 101, Callaway NE 68825, has a good description of this branch. This is an excellent book well worth having. Some highlights and partial quotes from the book, page 17:

-constructed in 1907-10 by the Iron Dyke Railroad and the Northwest Railway.
-turned over to the OSL (Oregon Short Line) for operation on July 1, 1910.
-ran northward 57.6 miles from Blakes Jct, 1.7 miles east of Huntington OR to Homestead OR

It was to have extended down the Snake River to Lewiston ID where it would have connected with the Camas Prairie RR and an outlet to the OWR&N. This would have proveded a water level route around the heavy grades of the Blue Mountains all the way to Portland. The probable reason for not completing the line through to Lewiston was the very high construction costs it would have entailed. Due to the rugged terrain, there were seven tunnels on the branch. After the copper mines in the Homestead area closed down, the 24.7 miles between Robinette and Homestead were abandoned in 1931. When Hells Canyon Dam was constructed downstream on the Snake River in 1957-58, the line was abandoned (1958) rather than relocate it above the rising backwaters.
ENDQUOTE

I have never been north of Huntington on the Snake, nor more than a few miles south of Lewiston. It is possible to travel by boat from Lewiston to the Hells Canyon dam, and likely one could also go downstream to the dam by boat from Huntington.

I suspect that much of this branch, especially the northern portions, is under water today. It is possible that the communities of Robinette and Homestead are also under water.