This family trapped in a gorge in California was found in a few hours after the distress note they put in a water bottle and dropped in the river was found downstream.

News article and video here:

https://abc7.com/message-in-a-bottle-helps-rescue-family-trapped-on-camping-trip/5530901/
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)