The current state trail route is built on an old highway detour graded for the construction of the US 30 "water grade" route...
I believe that this detour was created when the Bureau of Public Roads (now Federal Highway Administration) was constructing the new section of road from Bonneville to Cascade Locks, as required by construction of Bonneville Dam in 1936. This section included the Toothrock Tunnel, still used for I-84 eastbound. This section was the first part of the HCRH to be modified to become the “water grade” route (2-lane) then widened to become the Interstate (first named I-80N, then I-84). Yes, the detour would have been made to keep traffic flowing on the old highway while the big cut was being made for the new highway.
Some of the detours are shown on old ODOT maps and plans.