Where was The Johnstown Flood filmed?
1926 film by Irving Cummings
The Johnstown Flood (1926) was filmed in Santa Cruz, 1 place named in its Wikipedia article's Production section, in the United States. Wikidata records no filming location for it at all, so the article is the only recorded source. Each place says where its claim comes from.
Places per Wikipedia's Production section
Wikipedia’s Production section names 1 place. Each links to the sentence that says so.
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Towns & cities
Santa Cruz
United States
City in and county seat of Santa Cruz County, California, United States
“The flood sequences were actually filmed in and around Santa Cruz, California.” Wikipedia