This was followed by the railroad, mostly built on the route of the trail; a dirt road was needed for construction that was later converted into part of the present state highway.

Previously, the road was signed as U.S. Route 40 Alternate, crossing the Sierra Nevada at a lower elevation than Donner Pass on US 40, now Interstate 80 (I-80).

State Route 70 begins at a partial interchange with SR 99 north of Sacramento, close to the Feather River Route rail line that parallels the entire highway, and heads north along a four-lane mix of expressway and freeway.

After a fair distance through the canyon, and that formed by the East Branch North Fork Feather River, SR 70 reaches the junction with State Route 89 near Paxton; Routes 70 and 89 overlap southeast from that point, where the East Branch splits into Indian Creek and Spanish Creek.

[3] The highway heads southeast, partly along the latter creek, past Keddie to Quincy in the American Valley.

It leaves the valley via Greenhorn Creek, passing the Feather River Route's Williams Loop and then following the small Estray Creek to Lee Summit, which the rail line passes under in the Spring Garden Tunnel.

This brings SR 70 into the valley of the Middle Fork Feather River, which takes it southeast to Blairsden, where the State Scenic Highway ends and State Route 89 splits to the south, and then east, through the Plumas National Forest, to Portola and Beckwourth.

This followed the old Beckwourth Trail east of Quincy, but to the west it reached Oroville and Marysville via the Feather River Canyon.

While building the railroad, the Utah Construction Company had created a dirt road through the canyon to assist with construction; citizens created the Plumas County Road Association in 1911 to push for improvements to this roadway and creation of a year-round route between Oroville and Quincy (the existing route over the ridges was closed for at least four months each winter).

[13][14] Construction began on July 1, 1928, with convict labor for the easier portions and contractors for the remainder, as well as bridges and tunnels, but was slowed by the Great Depression.

On the most difficult portion, between Cresta and Rock Creek, three tunnels had to be built at Arch Rock, Grizzly Dome, and Elephant Butte; at the former two, surveyors had to hang out on rope over steep granite slopes, and rockslides repeatedly caused delays.

It left to the north on Dark Canyon Road, meeting the present alignment at Jarbo Gap.

[31] Three portions of SR 70 have been upgraded to freeways: north of SR 99 to Berry and Kempton Roads in the early 2010s; south from Marysville to the State Route 65 split in the mid-1950s, extended farther south in the late 1960s and late 2000s; and around downtown Oroville, built in the early 1960s.

[34][35][36] In the early 2010s, the last two-lane segment of SR 70 south of Marysville was expanded to a four-lane expressway, with a freeway section bypassing the small town of East Nicolaus to the west.

In 2015, the interchange with Feather River Boulevard in Plumas Lake was opened to traffic, eliminating the last signalized intersection between Sacramento (with SR 99) and Marysville.