(within Umatilla; MP 184.13-184.87, 0.74 miles)
"Over the Columbia River Highway from its junction with the Old Oregon Trail Highway, I-84 and US30, east of Boardman, easterly via Irrigon, Umatilla, and Cold Springs to the Washington State Line near Wallula, Washington (common with US395 from the junction with the McNary Highway in Umatilla easterly to its junction with the Umatilla-Stanfield Highway east of Umatilla)."
~ ODOT, Descriptions of US and Oregon Routes, March 2007
US-730 was created at the inception of the original United States Interstate Highway System (now just the US Route System) in 1926, running a paltry 18½ miles in Oregon from US-730 in Umatilla to the Washington Border along the Umatilla Cutoff #37. This was the era when US routes were just thrown about, regardless of length (the grand total length of the highway at this time was about 25½ miles in both states), and US-730 wasn't even an improved road. That all changed when the Umatilla Cutoff #37 was retired in favor of extending the Columbia River Highway #2 along US-730 in 1932 (coinciding with the start of the State Route System), when the road was improved in Oregon (it is unknown when the Washington portion was improved, but it was improved by 1941).
In 1945, a new portion of Oregon's Old Oregon Trail Highway #6 (designated as US-30) was built between Boardman and Pendelton. This moved US-30 and US-730's junction, formerly in Umatilla, about 19 miles away to Boardman; the old part of US-30/Old Oregon Trail became the Umatilla-Stanfield Highway #54 and was designated as OR-32. US-730's route length is now about 43 miles long in total, but in 1962 a plan materialized to extend US-12 from Lewiston, Idaho through Washington to Boardman, presumably along US-730 (which would've effectively retired it). This plan wasn't accepted, and in 1967 US-12 was completed to the coastal city of Aberdeen, Washington. One year earlier, US-730's routing was realigned between Boardman and Irrigon to a route a little more to the east (now meeting US-30 at Boardman Jct.), and it has been this way ever since.
Some portion of US-730 has been cosigned with US-395 since 1935. When US-395 was extended south from Spokane to San Diego, it was cosigned for almost 10 miles from Cold Springs Jct. to the Washington border (Washington's portion was completely cosigned). And it stayed that way until 1976, when US-395 was moved onto the Umatilla-Stanfield Highway and the Old Oregon Trail (then I-80N, now I-84), extending US-395's codesignation to 18.41 miles (all of US-730's original Oregon length). Finally, in 1987, when I-82 was completed between Ordnance, Oregon and Ellensburg, Washington, US-395 was rerouted onto I-82 in Washington and briefly in Oregon, shortening US-395's duplex to its current length to 0.74 miles.
The old portion of US-730 (and a little bit of old US-30) is still mostly intact from Boardman to Irrigon. This route, shown in red, followed Columbia Blvd. from Boardman and headed northeast roughly following the Columbia River banks to a slough that caused some of the old highway to be removed. This now-absent section met up with Paterson Ferry Rd. (Morrow County Route 930) before heading east (still currently Columbia Blvd.) to Irrigon. Then at Irrigon, it became S Main St. before rejoining the current US-730 in downtown Irrigon. Pre-1945, this section (as well as the section of current US-730 from Irrigon to Umatilla) was a part of US-30 before it was realigned roughly along current I-84. The gray road that old US-730 connects to is that realigned former US-30, which has been removed at both ends for the completion of I-84 through town. The western terminus is now at a park, and the eastern terminus now blends seamlessly with old US-730; however, as late as 1977 the old roadways were still being used as on- and off-ramps for Exit 165 I-84 (then I-80N) (according to a topographic map from 1977). The interchange was rebuilt as of 1991 (according to a topographic map from 1991).