San Gabriel River (California) History, where to find?

I want to metal detect in the San Gabriel Riverbed near Whittier Narrows Dam. I'm trying to piece together information about its history? In particular when the San Gabriel River Bicycle Path was built. So far I've learned when the Whittier Narrows Dam was constructed. I'm trying to get an idea of... show more I want to metal detect in the San Gabriel Riverbed near Whittier Narrows Dam. I'm trying to piece together information about its history?

In particular when the San Gabriel River Bicycle Path was built. So far I've learned when the Whittier Narrows Dam was constructed. I'm trying to get an idea of what type of area it was before construction took over. If people went over that path from the Old Spanish Trail off of the Santa Fe Trail.

Any ideas on where I can find out when the bike path was built or what the surrounding area was like 50+ years ago?
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