Lower Blackburn Grade Bridge, Historic bridge near Bridgeville, California.

The Lower Blackburn Grade Bridge is a 258-foot reinforced concrete through arch with a main span of about 150 feet over the Van Duzen River. Its arch carries inset panels and an architectural rail, and it is the only reinforced concrete through arch built along the Van Duzen River Highway.

John B. Leonard designed this bridge in 1925 as one of five structures built for California State Route 36 connecting Fortuna and Red Bluff. It emerged during a period when engineers were experimenting with new reinforced concrete techniques for river crossings.

The bridge carries the name of the Blackburn Grade, the steep, curving road it was built to bypass in 1925, and locals also call it the Van Duzen River Bridge or Mile 18 Bridge. It was listed on the National Register of Historic Places on 25 June 1981. The 1985 replacement span honors Silvio "Botchie" Santi, an Italian immigrant.

The bridge sits in a rural area west of Bridgeville requiring travel through secondary roads in Humboldt County. Visitors should check road conditions before driving and allow time to locate it off the main highway routes.

In the 1995 film Outbreak, mostly shot in nearby Ferndale, a helicopter chase along the Van Duzen River has the helicopters flying under the Lower Blackburn Grade Bridge, skimming the top of the water. The bridge had already been closed to traffic for a decade when it appeared on screen.