Lake Anza is not really a lake but actually is classified as a recreational swimming reservoir within the massive Tilden Regional Park, which is located in the Berkeley Hills above Berkeley, California. 

The lake was formed when the construction of the C.L. Tilden Park Dam was completed in 1938. It has a mix of grassy fields and some wooded hiking trails nearby. The recreational park area around it's vicinity is open year round but swimming is only allowed during the summer months. 

It has park facilities such as public restrooms, a sandy beach, picnic facilities, volleyball courts (seasonal),  a trail that circles around the entire reservoir and lots of ample parking.  Although there is posted warnings of possible mountain lions in the general area, I personally never felt unsafe or threatened at this particular location. Usually its pretty quiet and serene, if you like that sort of thing.