Lower Blackburn Grade Bridge
Also known as: 18-Mile Bridge; Van Duzen River Bridge; Bridge No.4-97
Why it's listed
Carrying State Highway 36 over the Van Duzen River near Bridgeville in Humboldt County, this 1925 bridge has a reinforced concrete through arch main span flanked by concrete slab approach spans. Its eight spans total 258 feet, and the two-lane roadway runs between concrete window-type railings. It was built to replace the hazardous Blackburn Grade, where steep grades, sharp curves and unstable slopes made winter travel dangerous. The county road improvement program that produced it hired San Francisco consulting engineer John B. Leonard, an early advocate of reinforced concrete bridges in California. Leonard designed only two through arch bridges, and this is the one that survives. The bridge keeps its original location, design, materials and workmanship, and reflects his attention to graceful form suited to the site. It was listed for its role in highway history, its link to Leonard, and its value as an example of mid-1920s concrete arch construction.
Significance in brief: The bridge is nominated under criteria A, B and C. It replaced the hazardous Blackburn Grade on the Fortuna-Red Bluff Highway and represents the work of John B. Leonard, a leading early 20th century designer of reinforced concrete bridges in California. It embodies a distinctive type, period and method of construction, and is the only remaining one of two through arch bridges known to have been built to his design.
Machine-generated summary. This summary was generated by a language model from the official nomination document and has not been reviewed line by line by an editor. It may contain errors. Check the source document before relying on any detail. Source: nomination document (PDF) · National Archives catalog entry. NRHP reference 81000148, listed June 25, 1981. Summary generated October 2026 (model claude-sonnet-5-5, prompt v1.0-2026-10-29, confidence high).
Key facts
| Address | NW of Bridgeville on CA 36, Bridgeville |
|---|---|
| County | Humboldt County |
| Listed | June 25, 1981 |
| Reference number | 81000148 |
| Resource type | Structure |
| Period of significance | 1900- (specific date 1925) |
| Architect / builder | ENGINEER: John B. Leonard |
| Architectural style | Reinforced concrete through arch bridge |
| Criteria |
|
| Areas of significance | Engineering, Transportation, Architecture |
| Level of significance | Local |
| Status (as stated in the nomination) | Listed as occupied, publicly owned, and in use for transportation; the form marks it unaltered, on its original site, and in good condition. |
| Acreage | 0.9 |
| Coordinates | 40.48055, -123.89060 (NPS GIS) |
What listing means for this property
For a publicly owned building or structure, listing mainly matters when federal funds, permits or licences are involved: projects then go through Section 106 review so effects on the property are considered. Listing also strengthens applications for preservation grants and, for leased income-producing uses, can enable the federal rehabilitation tax credit.
General information, not legal or tax advice. Rules change; confirm with your State Historic Preservation Office or a qualified adviser.
Sources and further reading
- National Park Service, National Register of Historic Places database (snapshot May 2026), reference 81000148.
- Nomination form (PDF, National Archives) — catalog entry
- Wikipedia article (used for cross-checking only; text not reproduced here)
- Wikidata Q6693324
- This listing as JSON
Last updated October 2026. Found an error? See the editorial policy for how to report it.