the Marines using 55 Chevies as rip rap in San Gabriel in 1969 when the river flooded

Might be the Eaton Wash, Rubio Wash, or the Alhambra Wash... or the Rio Hondo

https://authors.library.caltech.edu/25781/3/TR000048.pdf
https://www.mopar-forum.eu/viewtopic.php?f=22&t=9064&start=2740
https://www.insidetheie.com/inland-empire-floods
https://www.palisadeshudson.com/2014/08/drought-stricken-california-ponders-an-arkstorm/
http://ochistorical.blogspot.com/2010/01/flood-of-1969-hb-lifeguards-bungalows.html