This photo of the Buena Vista Reservoir from the January 24, 1909,
Los Angeles Herald is captioned, "Buena Vista Reservoir, beautiful sheet of placid water
hidden away in Elysian Park. This is a favorite spot for motorists who seek an hour of delightful seclusion."
The bridge and tower in the lower right corner, and the shape of the lake and the hill behind it, look like
e_r's ebay photo above:
Newspapers.com
Here is the reservoir empty and next to the northbound Pasadena Freeway in 1956. I guess that's the old caretaker's house near the upper right corner:
March 16, 1956,
Los Angeles Mirror @ Newspapers.com