The next stop on the Columbia Gorge bridge project is not exactly a bridge; the old Columbia River Highway travels on raised viaducts for a few hundred feet on either side of Multnomah Falls, for the simple reason that there was nowhere else to put a road back then, as the only bit of flat ground was already taken up by the railroad line. (The area where I-84 runs now was filled in much later, and was just river or wetland a century ago). Like most bridges on this stretch of highway, the viaducts were designed by K.P. Billner, who wrote about them in a 1915 article:
Two long concrete viaducts, which stand against the hillside like steps, solved the problems existing near Multnomah Falls. Here the Oregon-Washington Railway & Navigation Co.’s road occupied all of the available space along the river. The cost of excavating a 24-ft. roadway along this railroad, and of carrying the excavated material across the track to the river, was practically prohibitive. Figure is a view of the 860-ft. viaduct located east of Multnomah Falls. The West Multnomah Viaduct is similar to that shown in Fig. 6. In the West Multnomah Viaduct, however, the the railing extends along the masonry retaining wall.
The roadways of these viaducts are carried on two parallel lines of columns spaced 17 ft. 6 ins. on centers. The longitudinal spacing of these columns is 20 ft. As protection agains possible settlement of the upper columns, inclined struts, following the slope of the hill, are placed between the upper and lower columns, these struts being capable of carrying the weight of the structure. A railing, consisting of cement mortar on metal lath reinforcement, is placed along the railroad side only (see Fig 6.).
A page about the east viaduct at Recreating the Historic Columbia River Highway includes a few vintage photos, and notes that the east viaduct cost $22,520.83 in 1915 dollars, making it the second most expensive structure on the highway, second only to the Latourell Creek bridge. The east viaduct also has a BridgeHunter page, and the Library of Congress has a Historic American Engineering Record entry for it with a few old photos, which are mostly interesting because they're taken from angles you physically can't get to anymore, unless you feel like standing in the middle of eastbound I-84. Legal says I have to remind people not to do that, btw. The photos you see here were taken from the far eastern tip of the I-84 Multnomah Falls parking lot/rest area, which I think is probably the only way you can see underneath the structure now without being in a moving vehicle.
Given all that went into creating the Multnomah viaducts, it's a shame they're contenders for everyone's least favorite part of the old highway. They were engineering marvels of the Model T era, but they weren't designed with wider vehicles in mind, and it seems like there's always a giant RV with extra-wide side mirrors heading the other direction whenever you drive across one of the viaducts. In the last 10 years or so, it's also become rather likely you'll be stuck in a huge traffic jam all the way through the Multnomah Falls area, which doesn't really enhance the viaduct experience either. Widening the road is probably out; the viaducts are protected historic structures, and even if they weren't the rail line is still right next door & there's nowhere to put a wider road. I suspect that at some point they'll have to ban private vehicles along this part of the road, at least during high tourist season, and only allow shuttle buses along the road, sort of like what the National Park Service ended up doing in Yosemite National Park. This won't happen anytime soon, but in the longer term it seems inevitable to me. Then the shuttle drivers can tell their passengers the scary narrow part is coming up, and people can wave at the shuttle going the other direction, just inches away, and it'll just be another fun part of the show.
