Doug McIntosh kindly supplied the 1908 map below, which shows various waterways in Santa Ana Canyon. I wish I had a good way to overlay this map over a modern Thomas Guide.
In the photo below, Steve Faessel inspects and old bridge we stumbled across in the brush. It's hard to say anymore exactly what it once bridged.
This area isn't far from the site of the old Rancho Santa Ana Botanic Gardens (RSABG), which is shown in the two vintage photos below. In fact, our "base camp" for this field trip was in front of the ranch house of Susanna Bixby-Bryant who founded the gardens in 1927. She was the daughter of John Bixby, who purchased the ranch property from Bernardo Yorba's widow in the 1870s.
The RSABG spanned 200 acres and specialized in native flora. Its Assembly Hall featured a large mission-style tower. Around the time of Bixby-Bryant's death, in the early 1950s, the Gardens were moved to Claremont. The ranch itself stayed in the family until 1978. Eventually, much of it was turned into tract housing.
I believe you can see some of our hiking route in the lowlands shown on the right side of the photo above.
Once again, thanks to Don, Steve, Carl, Ron, and everyone else who made this a fascinating trek.