From Wikipedia:

Approximately 7,500 extras worked on the film. During the filming of the climactic flood scene, the 600,000 US gallons (2,300,000 L; 500,000 imp gal) of water used was so overwhelming that three extras drowned, one was so badly injured that his leg needed to be amputated, and a number suffered broken limbs and other serious injuries, which led to implementation of stunt safety regulations the following year....Thirty-five ambulances attended the wounded.

I've seen this movie (which I rate 6/10) multiple times throughout the years and can attest proceedings during the flood portion do definitely look like they'd have been likely quite dangerous for participants.

By the way, for those who don't already know, star Dolores Costello was star Drew Barrymore's grandmother.

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