Here is is....
It's called the Los Angeles County Arboretum and Botanic Garden....
http://en.wikipedia.org/wiki/Los_Ang...Botanic_Garden
http://employees.oxy.edu/jerry/arbor.htm
On the westward side of the lake is the Queen Anne Cottage, built in 1885, which was used for exteriors in the television series Fantasy Island. In fact, the lake was used on the series where you see the incoming airplane taxiing in with the passengers for that week's show.
I believe this is near where they filmed MASH too.