Most of Franklin Canyon was owned by the Doheny family until 1977 - now it is part of the Santa Monica Mountains National Recreation Area. The information on its current configuration and facilities is here. It was saved from development in the seventies. Activists convinced the Los Angeles Department of Water and Power and the National Park Service that it shouldn't be subdivided into homes for the very rich.
It was Los Angeles Department of Water and Power land, really. In 1914, William Mulholland, the man who developed the complex water systems for the DWP (see Chinatown), began construction of the reservoirs in Franklin Canyon to provide the main water service for downtown and West Los Angeles. The Upper Reservoir, the lake, was intended only to provide stability to the main facility and electric generating plant at the lower reservoir. By 1916, both reservoirs were in operation. After the 1971 Sylmar earthquake the strength of the reservoir system was questioned, and studies were done to asses the danger. They decided to take both the upper and lower reservoirs out of service and build a single, more modern and stable reservoir facility a quarter mile north of what was then the lower reservoir. The new rubber-covered facility is the Franklin 2 reservoir - in operational since 1982. It's ugly. The rest is now a park.
There's a good history here, although it concentrates on the more than thirty episodes of the 1962-1967 television show Combat! that were filmed here - no palm trees, so it looked like Europe in the forties, or close enough. You also may have seen the lake in into opening titles for the old Andy Griffin Show - Opie and Andy walking to the fishing hole. This stuff goes way back. In the 1930s the movie industry made arrangements with DWP to use the area for filming. It was an ideal site, close to the studios but far from any urban development, and completely off-limits to the public. Claudette Colbert's famous hitch-hiking scene in "It Happened One Night" was filmed here in 1935. The site is still being used, almost daily, by the film and television industry, although much of that is done far from the public areas. And a curiosity - the album covers for the Rolling Stones' album "Big Hits (High Tide and Green Grass)" and Simon and Garfunkel's "Sounds of Silence" were both shot here.
A few photos of the place were posted earlier here (July 13, 2003) and here (December 28, 2003).