{"nrhp_refnum":"81000148","name":"Lower Blackburn Grade Bridge","other_names":["18-Mile Bridge","Van Duzen River Bridge","Bridge No.4-97"],"category":"structure","location":{"address":"NW of Bridgeville on CA 36","address_restricted":false,"city":"Bridgeville","county":"Humboldt County","state":"CA","lat":40.480548371000054,"lon":-123.89059651899998,"geom_source":"nps_gis"},"listed_date":"1981-06-25","is_nhl":false,"nhl_designated_date":null,"areas_of_significance":["Engineering","Transportation","Architecture"],"significance_levels":["local"],"summary":{"what_it_is":"A reinforced concrete through arch highway bridge carrying California State Highway 36 across the Van Duzen River near Bridgeville, Humboldt County, California, built in 1925 to a design by engineer John B. Leonard.","why_significant":"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.","narrative":"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.","period_of_significance":"1900- (specific date 1925)","architect_builder":"ENGINEER: John B. Leonard","architectural_style":"Reinforced concrete through arch bridge","criteria":["A","B","C"],"status_note":"Listed as occupied, publicly owned, and in use for transportation; the form marks it unaltered, on its original site, and in good condition.","vacant_flag":false,"machine_generated":true,"model":"claude-sonnet-5-5","prompt_version":"v1.0-2026-10-29","generated_at":"2026-10-29 02:55:31.638376-05","source_pdf":"https://s3.amazonaws.com/NARAprodstorage/lz/electronic-records/rg-079/NPS_CA/81000148.pdf"},"photo":{"url_800":"https://nationalhistoricalregister.com/media/81000148/e06efe7551-800.webp","url_1600":"https://nationalhistoricalregister.com/media/81000148/e06efe7551-1600.webp","licence":"CC BY-SA 4.0","author":"Carlos Arturo","credit_html":"<a href=\"https://commons.wikimedia.org/wiki/File:Lower_Blackburn_Grade_Bridge.jpg\" rel=\"noopener\">Carlos Arturo</a>, <a href=\"https://creativecommons.org/licenses/by-sa/4.0\" rel=\"license noopener\">CC BY-SA 4.0</a>, via Wikimedia Commons","source_page":"https://commons.wikimedia.org/wiki/File:Lower_Blackburn_Grade_Bridge.jpg"},"district":null,"sources":{"nps_snapshot_date":"2026-05-22","nara_catalog":"https://catalog.archives.gov/id/123858770","wikipedia":"https://en.wikipedia.org/wiki/Lower_Blackburn_Grade_Bridge","wikidata":"https://www.wikidata.org/wiki/Q6693324"},"indexable":true,"url":"https://nationalhistoricalregister.com/california/humboldt-county/lower-blackburn-grade-bridge-81000148/"}