The highway originates at the north end of the Astoria–Megler Bridge west of Megler at an intersection with U.S. Route 101 (US 101).
The roadway was extended west to the north end of the bridge, where it currently terminates.
The former ferry terminal became the Dismal Nitch rest area, named after the Lewis and Clark Expedition's description of the place.
SR 401 originates at an intersection with U.S. Route 101 (US 101) at the north end of the Astoria–Megler Bridge west of Megler and southeast of McGowan.
Traveling northeast between cliffs and the Columbia River past the Dismal Nitch Safety rest area to Knappton,[1] the highway turns north along more cliffs and the South Fork of the Naselle River.